mc_cursor finishes returning before stopping

[samwar]

Prior to this, mc_cursor processes would shut down after returning the last result, rather than after
returning a final 'empty' to callers. This caused callers to try to call back for 'next()' again when
cursor was already down or going down. That call is a gen_server:call(). In the best case, the process
would already be down, and the call would result in a noproc exit, which this code catches and
translates into a return value of 'error', instead of the correct {} value. In rare cases, though,
due to gen_server code, the call would occur at the moment the process terminates, and you'll get
an exit(normal) on the calling process instead.