rpc: Report back first error message on failure

When running criu in swrk mode the client typically wants to know
the reason of failure. Right now criu reports back NOTHING but the
fact that dump/restore/etc fails. We've tried to address this by
introducing the cr-errno engine, but it doesn't seem to be informative
enough and is hard to maintain -- adding new errno-s is boring :(

I propose to report back the first message with ERROR level upon
failrure as __typically__ the very first error message indicates
that proceeding is impossible and criu rolls back (generating more
error messages, so it's crucial to know the very first one).

If we ever meet the situation that the first pr_err/pr_perror doesn't
cause criu to exit, this printing should be fixed to be pr_warn.

Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Acked-by: Andrei Vagin <avagin@virtuozzo.com>