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,