localedef: add newlines to error messages

These won't be added elsewhere, so add a little bit of room to make
these messages a little easier to read. The existing set is a mixed
bag, there are somewhere in the ballpark of 45, 46 printfs to stderr and
19 of those had newlines.

Reviewed by: yuripv
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D41693