This change makes this module act more like most of our other low-level
resource management modules.  It's a caller error if something is not
explicitly closed by the end of a successful transaction, so issue
a WARNING about it.  This would not actually have caught the file leak
bug fixed in commit 231bcd08, because that was in a transaction-abort
path; but it still seems like a good, and pretty cheap, cross-check.

Discussion: https://postgr.es/m/152056616579.4966.583293218357089052@wrigleys.postgresql.org