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.

NEWS

@@ -4,6 +4,16 @@ GNU Bison NEWS
 
 ** Bug fixes
 
+*** Generated source files when errors are reported
+
+  When warnings are issued and -Werror is set, bison would still generate
+  the source files (*.c, *.h...).  As a consequence, some runs of "make"
+  could fail the first time, but not the second (as the files were generated
+  anyway).
+
+  This is fixed: bison no longer generates this source files, but, of
+  course, still produces the various reports (*.output, *.xml, etc.).
+
 *** %empty is used in reports
 
   Empty right-hand sides are denoted by '%empty' in all the reports (text,