Add message to formatter SyntaxError (#5881)

**Summary** Add a static string error message to the formatter syntax
error so we can disambiguate where the syntax error came from

**Test Plan** No fixed tests, we don't expect this to occur, but it
helped with transformers syntax error debugging:

```
Error: Failed to format node

Caused by:
    syntax error: slice first colon token was not a colon
```

crates/ruff_formatter/src/diagnostics.rs

@@ -8,7 +8,7 @@ use std::error::Error;
 pub enum FormatError {
     /// In case a node can't be formatted because it either misses a require child element or
     /// a child is present that should not (e.g. a trailing comma after a rest element).
-    SyntaxError,
+    SyntaxError { message: &'static str },
     /// In case range formatting failed because the provided range was larger
     /// than the formatted syntax tree
     RangeError { input: TextRange, tree: TextRange },
@@ -28,7 +28,9 @@ pub enum FormatError {
 impl std::fmt::Display for FormatError {
     fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
-            FormatError::SyntaxError => fmt.write_str("syntax error"),
+            FormatError::SyntaxError {message} => {
+                std::write!(fmt, "syntax error: {message}")
+            },
             FormatError::RangeError { input, tree } => std::write!(
                 fmt,
                 "formatting range {input:?} is larger than syntax tree {tree:?}"
@@ -57,6 +59,12 @@ impl From<&PrintError> for FormatError {
     }
 }
 
+impl FormatError {
+    pub fn syntax_error(message: &'static str) -> Self {
+        Self::SyntaxError { message }
+    }
+}
+
 #[derive(Debug, Copy, Clone, Eq, PartialEq)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub enum InvalidDocumentError {

crates/ruff_python_formatter/src/comments/format.rs

@@ -251,7 +251,9 @@ impl Format<PyFormatContext<'_>> for FormatComment<'_> {
         let trailing_whitespace_len = comment_text.text_len() - trimmed.text_len();
 
         let Some(content) = trimmed.strip_prefix('#') else {
-            return Err(FormatError::SyntaxError);
+            return Err(FormatError::syntax_error(
+                "Didn't find expected comment token `#`",
+            ));
         };
 
         // Fast path for correctly formatted comments:

crates/ruff_python_formatter/src/expression/expr_slice.rs

@@ -162,18 +162,22 @@ pub(crate) fn find_colons(
     let after_lower = lower
         .as_ref()
         .map_or(range.start(), |lower| lower.range().end());
-    let first_colon =
-        first_non_trivia_token(after_lower, contents).ok_or(FormatError::SyntaxError)?;
+    let first_colon = first_non_trivia_token(after_lower, contents).ok_or(
+        FormatError::syntax_error("Din't find any token for slice first colon"),
+    )?;
     if first_colon.kind != TokenKind::Colon {
-        return Err(FormatError::SyntaxError);
+        return Err(FormatError::syntax_error(
+            "slice first colon token was not a colon",
+        ));
     }
 
     let after_upper = upper
         .as_ref()
         .map_or(first_colon.end(), |upper| upper.range().end());
     // At least the closing bracket must exist, so there must be a token there
-    let next_token =
-        first_non_trivia_token(after_upper, contents).ok_or(FormatError::SyntaxError)?;
+    let next_token = first_non_trivia_token(after_upper, contents).ok_or(
+        FormatError::syntax_error("Din't find any token for slice second colon"),
+    )?;
     let second_colon = if next_token.kind == TokenKind::Colon {
         debug_assert!(
             next_token.range.start() < range.end(),

crates/ruff_python_formatter/src/expression/string.rs

@@ -91,7 +91,9 @@ impl Format<PyFormatContext<'_>> for FormatStringContinuation<'_> {
                     continue;
                 }
                 Err(_) => {
-                    return Err(FormatError::SyntaxError);
+                    return Err(FormatError::syntax_error(
+                        "Unexpected lexer error in string formatting",
+                    ));
                 }
             };
 
@@ -167,7 +169,9 @@ impl Format<PyFormatContext<'_>> for FormatStringPart {
         let prefix = StringPrefix::parse(string_content);
         let after_prefix = &string_content[usize::from(prefix.text_len())..];
 
-        let quotes = StringQuotes::parse(after_prefix).ok_or(FormatError::SyntaxError)?;
+        let quotes = StringQuotes::parse(after_prefix).ok_or(FormatError::syntax_error(
+            "Didn't find string quotes after prefix",
+        ))?;
         let relative_raw_content_range = TextRange::new(
             prefix.text_len() + quotes.text_len(),
             string_content.text_len() - quotes.text_len(),

crates/ruff_python_formatter/src/statement/stmt_assign.rs

@@ -20,7 +20,9 @@ impl FormatNodeRule<StmtAssign> for FormatStmtAssign {
             type_comment: _,
         } = item;
 
-        let (first, rest) = targets.split_first().ok_or(FormatError::SyntaxError)?;
+        let (first, rest) = targets.split_first().ok_or(FormatError::syntax_error(
+            "Expected at least on assignment target",
+        ))?;
 
         write!(
             f,

crates/ruff_python_formatter/src/statement/stmt_with.rs

@@ -117,14 +117,19 @@ fn are_with_items_parenthesized(
     with: &AnyStatementWith,
     context: &PyFormatContext,
 ) -> FormatResult<bool> {
-    let first_with_item = with.items().first().ok_or(FormatError::SyntaxError)?;
+    let first_with_item = with
+        .items()
+        .first()
+        .ok_or(FormatError::syntax_error("Expected at least one with item"))?;
     let before_first_with_item = TextRange::new(with.start(), first_with_item.start());
 
     let mut tokenizer = SimpleTokenizer::new(context.source(), before_first_with_item)
         .skip_trivia()
         .skip_while(|t| t.kind() == TokenKind::Async);
 
-    let with_keyword = tokenizer.next().ok_or(FormatError::SyntaxError)?;
+    let with_keyword = tokenizer.next().ok_or(FormatError::syntax_error(
+        "Expected a with keyword, didn't find any token",
+    ))?;
 
     debug_assert_eq!(
         with_keyword.kind(),