lvmetad: handle update failures

lvmetad: handle update failures

If a command gets stuck during an lvmetad update, lvmetad
will cancel that update after the timeout.  The next command
to check the lvmetad will see that lvmetad needs to be
populated because lvmetad will return token of "none" after
a timed out update (same as when lvmetad is not populated
at all after starting.)

If a command gets an error during an lvmetad update, it
will now just quit and leave its updating token in place.
That update will be cancelled after the timeout.