Modify heap_open()/heap_openr() API per pghackers discussion of 11 July.

These two routines will now ALWAYS elog() on failure, whether you ask for
a lock or not.  If you really want to get a NULL return on failure, call
the new routines heap_open_nofail()/heap_openr_nofail().  By my count there
are only about three places that actually want that behavior.  There were
rather more than three places that were missing the check they needed to
make under the old convention :-(.