Fix pg_restore so parallel restore doesn't fail when the input file doesn't

Fix pg_restore so parallel restore doesn't fail when the input file doesn't
contain data offsets (which it won't, if pg_dump thought its output wasn't
seekable).  To do that, remove an unnecessarily aggressive error check, and
instead fail if we get to the end of the archive without finding the desired
data item.  Also improve the error message to be more specific about the
cause of the problem.  Per discussion of recent report from Igor Neyman.

Back-patch to 8.4 where parallel restore was introduced.