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 {