vm: call finish callback always

Always call the finish callback to make control flow consistent
if VM crash/does not crash. Then users can rely on the callback
being always called.

Fix a bug highlighted by the extended test:
currently we call extractError/callback twice when the fuzzer is preempted.
If the fuzzer is preempted, extractError returns nil,
which makes appendOutput return nil as well,
which makes the main loop continue as if no crash/preemption happened.
It will exit, but only after 5 min "no output" timeout.
Most likley the output will still contain the preemption message,
so no "no output" will be reported, but the additional 5 min wait
is unnecessary.

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