Catch IndexError in DateTimeParser.parse for malformed format strings

[karthiksai109]

When a malformed format string with certain bracket patterns is passed to DateTimeParser.parse, the bracket-escaping logic in _generate_pattern_re can strip out named groups from the final regex while still keeping those tokens in the token list. This causes match.group(token) to raise an IndexError instead of a proper ParserMatchError.

This wraps the match.group() calls in a try/except so that an IndexError from a missing group gets converted into a ParserMatchError, consistent with how other parse failures are reported.

Includes a regression test using the format string from the original report.

Fixes #1191

[codecov]

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Additional details and impacted files

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  Misses            ?         0           
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