output: do not generate source files when early errors are caught

output: do not generate source files when early errors are caught

Reported by Alexandre Duret-Lutz as "second problem" in:
http://lists.gnu.org/archive/html/bug-bison/2013-09/msg00015.html

One problem is that some errors are caught early, before the
generation of output files, while others can only be detected
afterwards (since, for instance, skeletons can raise errors
themselves).

This will be addressed in two steps: early errors do not generate
source files at all, while later errors will remove the files that
have already been generated.

* src/scan-skel.l (yyout): Open to /dev/null when there are errors.
* tests/output.at (AT_CHECK_FILES): Factored out of...
(AT_CHECK_OUTPUT): this.
Fuse the "SHELLIO" argument in the "FLAGS" one.
Use $5 to denote the expected exit status.
Add a test case for early errors.