FREEPBX-7923 - Allow splice to add to an empty exten

Also, it complains loudly if something tries to splice into
an exten with a priority higher than 1, as it'll break asterisk
in strange and confusing ways. So don't.
Use log( instead of out(, add i18n if anyone ever wants it.
Merge branch 'bugfix/FREEPBX-7923' into develop
throw exception here