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/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(),