Check and dump the signal info for killed updater.

WEXITSTATUS only gives meaningful value if WIFEXITED is true. So we may
see a slightly confusing message of "E:Error in /path/to/package (Status
0)" for a killed updater process (e.g. updater calling abort(3)). This
CL dumps the signal number for such a case.

Test: abort() in child process. Check the output.
Change-Id: I72e6654a6499d65155085de658062efb9f1e36ac