xgettext: Clarify some diagnostics as errors vs. warnings.
* gettext-tools/src/x-csharp.c (phase2_getc): Clarify role of diagnostics.
* gettext-tools/src/x-javascript.c (phase2_getc): Likewise.
* gettext-tools/src/x-perl.c (get_here_document, extract_balanced): Likewise.
* gettext-tools/src/x-python.c (phase2_getc): Likewise.
* gettext-tools/src/x-rst.c (extract_rst, extract_rsj): Likewise.
* gettext-tools/src/xg-arglist-parser.c (arglist_parser_done): Likewise.
