Add failpoint to skip doing retries on WriteConflictExceptions

An insert or update operation retries ad infinitum on a unique key violation that results from the effects of an active transaction (read: a transaction not yet committed or aborted). This behavior is demonstrated by the write_conflicts_with_non_txns.js test. In order to allow the rollback fuzzer to generate randomized insert and update operations that may trigger unique key violations without hanging, it would be useful to add a failpoint to the writeConflictRetry() function where it doesn't do any retry logic and instead has the command fail with a WriteConflict error response.