always check return value of close_tempfile

always check return value of close_tempfile

If close_tempfile() encounters an error, then it deletes the
tempfile and resets the "struct tempfile". But many code
paths ignore the return value and continue to use the
tempfile. Instead, we should generally treat this the same
as a write() error.

Note that in the postimage of some of these cases our error
message will be bogus after a failed close because we look
at tempfile->filename (either directly or via get_tempfile_path).
But after the failed close resets the tempfile object, this
is guaranteed to be the empty string. That will be addressed
in a future patch (because there are many more cases of the
same problem than just these instances).

Note also in the hunk in gpg-interface.c that it's fine to
call delete_tempfile() in the error path, even if
close_tempfile() failed and already deleted the file. The
tempfile code is smart enough to know the second deletion is
a noop.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>