handle libfuzzer targets crashing without dropping an a crashing input

[bmc-msft]

When a libfuzzer target crashes in a fashion that libfuzzer can't handle, such as raise(SIGKILL), the input isn't saved to disk.

We should report this behavior as a failure to the user, so they can fix their libfuzzer target to not crash in such a fashion.

Basic logic:

if task == libfuzzer_fuzz and exit_code != 0 and get_crash() == None {
   mark task as failed
}