ConcurrentMirroringBUfferedMutator mutate() doesnt always rethrow exceptions

This causes a flaky test in TestConcurrentMirroringBufferedMutator#testErrorsReportedBySecondaryAreReportedAsWriteErrors. This can easily be reproduced by running the test repeatedly until failure.

Example failure: https://source.cloud.google.com/results/invocations/f5400b0a-852b-4a7f-88c2-d9aabf0a05e9/log

Start with TestConcurrentMirroringBufferedMutator#testErrorsReportedBySecondaryAreReportedAsWriteErrors and reproduce the failure by running it repeatedly. Review ConcurrentMirroringBufferedMutator.mutate() and the linked failure log; done means the test consistently reports secondary exceptions as write errors without flaky failures.

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