omcache.py: handle missing responses gracefully
omcache.py: handle missing responses gracefully

omcache_command() and omcache_io() may return without responses if the
request times out (or if we have a bug in omcache.c).  Handle these
gracefully by raising NotFoundError in get() and CommandError in all other
cases instead of failing with IndexError in list()[0].

thedrow/omcache,ohmu/omcache,CrowdStrike/omcache,CrowdStrike/omcache,thedrow/omcache,ohmu/omcache