Fix some error-reporting macro bugs.

Fix some error-reporting macro bugs.
* data/bison.m4 (b4_cat): New.
(b4_error, b4_error_at): Use b4_cat to send error directives directly
to stdout so they don't become arguments to other macros.  Update
comments and add examples.
(b4_warn, b4_warn_at, b4_complain, b4_complain_at): Update comments and
add examples.
(b4_fatal, b4_fatal_at): Likewise, and invoke m4_exit(1) immediately
after printing the error directive so that M4 doesn't report subsequent
problems that are induced by this problem.
* src/scan-skel.l: Recognize @` digraph outside of directive arguments
instead of just in them.  Recognize @\n in both places.  Both expand to
the empty string.  Needed by b4_cat.
* tests/skeletons.at (Complaining during macro argument expansion):
New test case.
(Fatal errors make M4 exit immediately): New test case.