run_phase: explicit handling of process.returncode values

[d-hat]

run_phase() has stanzas for process.returncode in [0, 1, >=100] but other values fall through without any action or logging. For robustness it's probably wise to catch other values and log a message. Particularly process.returncode<0 which indicates the subprocess was killed by a signal.

[kylape]

Sounds like a good thing to do.