Add newline after multimatch string error message
Add newline after multimatch string error message

Other utility functions, such as caller.msg(), don't require adding
newline because newlines are added implicitly.  This change modifies the
multimatch string parameter to also not require newline, but add one
implicitly.

jamesbeebop/evennia,jamesbeebop/evennia,jamesbeebop/evennia