pgsql: Improve tuplestore's error messages for I/O failures.

Improve tuplestore's error messages for I/O failures.

We should report the errno when we get a failure from functions like
BufFileWrite.  "ERROR: write failed" is unreasonably taciturn for a
case that's well within the realm of possibility; I've seen it a
couple times in the buildfarm recently, in situations that were
probably out-of-disk-space, but it'd be good to see the errno
to confirm it.

I think this code was originally written without assuming that
the buffile.c functions would return useful errno; but most other
callers *are* assuming that, and a quick look at the buffile code
gives no reason to suppose otherwise.

Also, a couple of the old messages were phrased on the assumption
that a short read might indicate a logic bug in tuplestore itself;
but that code's pretty well tested by now, so a filesystem-level
problem seems much more likely.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/6554656ea2043c5bb877b427237dc5ddd7c5e5c8

Modified Files
--------------
src/backend/utils/sort/logtape.c    |    5 +--
src/backend/utils/sort/tuplestore.c |   64 +++++++++++++++++++++++++----------
2 files changed, 47 insertions(+), 22 deletions(-)