Instead of crashing dump/page-server when a problem detected after the
page-pipe was split, print nice error messages and return error.

travis-ci: success for page-xfer: page_server_get_pages: replace BUG_ONs with 'return -1' (rev2)
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>