msgcat: Signal error when the same msgid is used with and without msgid_plural.

msgcat: Signal error when the same msgid is used with and without msgid_plural.

Suggested by Mikko Rantalainen <mikko.rantalainen@peda.net>
in <https://lists.gnu.org/archive/html/bug-gettext/2019-09/msg00020.html>.

* gettext-tools/src/msgl-cat.c (catenate_msgdomain_list): Signal an error when
the same msgid is used with and without msgid_plural.
* gettext-tools/src/msgcat.c (main): Exit with failure code if there was an
error.
* gettext-tools/src/msgcomm.c (main): Likewise.
* gettext-tools/src/msguniq.c (main): Likewise.
* gettext-tools/tests/msgcat-21: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add it.