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_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(),