fix(errors): handle parse.ParseError without IndexError in FormatErrorMsg

[jdymitarai]

Summary

Fixes #1303, #1250, #1215, #1256 where YAPF crashed with an unhandled internal IndexError: tuple index out of range whenever parsing raised a parse.ParseError instead of producing a clean error diagnostic.

Cause & Analysis

In yapf/pytree/pytree_utils.py, when parsing fails, ast.parse(code) is executed to check if the snippet is invalid Python syntax (raising SyntaxError). If ast.parse succeeds (for example on newer language constructs or syntax supported by Python's parser but not by the lib2to3 grammar), ParseCodeToTree re-raises parse.ParseError.

When handling this exception in yapf_api.py, FormatErrorMsg(e) attempted to format the error location using e.args[1][0], e.args[1][1], e.args[1][2], assuming e.args[1] was a 3-element tuple (filename, lineno, column). However, _ylib2to3.pgen2.parse.ParseError.args is a 1-tuple ("bad input: ...",) where location metadata is stored in e.context ((prefix, (lineno, column))). This resulted in e.args[1] raising an unhandled IndexError: tuple index out of range instead of reporting the syntax error cleanly.

Solution

1. Explicitly recognize parse.ParseError in FormatErrorMsg:
   • Extract lineno and column from e.context[1] if available.
   • Extract the error message from e.msg.
2. Consistently use getattr(e, 'filename', None) across formatters.
3. Add a safe fallback block in FormatErrorMsg to handle unexpected exception argument shapes cleanly without raising IndexError, TypeError, or AttributeError.
4. Add unit tests for ParseError formatting and FormatErrorMsg in yapftests/yapf_test.py.