Make use of new error context stack mechanism to allow random errors

Make use of new error context stack mechanism to allow random errors
detected during buffer dump to be labeled with the buffer location.
For example, if a page LSN is clobbered, we now produce something like
ERROR:  XLogFlush: request 2C000000/8468EC8 is not satisfied --- flushed only
to 0/8468EF0
CONTEXT:  writing block 0 of relation 428946/566240
whereas before there was no convenient way to find out which page had
been trashed.