When running libfuzzer in 'fuzzing' mode, we expect the following on exit.
If the exit code is zero, crashing input isn't required. This happens if the user specifies '-runs=N'
If the exit code is non-zero, then crashes are expected. In practice, there are two causes to non-zero exits.
1. If the binary can't execute for some reason, like a missing prerequisite
2. If the binary _can_ execute, sometimes the sanitizers are put in such a bad place that they are unable to record the input that caused the crash.
This PR enables handling these two non-zero exit cases.
1. Optionally verify the libfuzzer target loads appropriately using `target_exe -help=1`. This allows failing faster in the common issues, such a missing prerequisite library.
2. Optionally allow non-zero exits without crashes to be a warning, rather than a task failure.
