Let processor AttributeErrors surface

[Bolt4243]

Custom data processors can raise AttributeError from their own code. _run_processor currently catches that along with a missing hook, so parsing continues after hiding the processor failure.

Look up the optional processor first, then call it outside the exception handler. Missing hooks remain optional, while errors raised by existing processors now reach the caller.

Tests: full unittest suite, workflow flake8 checks, and coverage run with 93% total coverage.