Fix Stackdriver deduplication for corpus minimization errors

[google-labs-jules]

Modified corpus_pruning_task.py to log detailed minimization errors as warnings and raise a generic CorpusPruningError. This prevents Stackdriver from creating new error groups for every unique fuzzer error message. Added a regression test case.

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PR created automatically by Jules for task 3514744668939488966 started by @jonathanmetzman

[google-labs-jules]

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