notmuch.c: add missing trailing newlines in two error messages

notmuch.c: add missing trailing newlines in two error messages

Removed duplicate error check (and the message) in 3rd case where
the same error message (w/o trailing newline) was present.

In case of test/T040-setup.sh, command substitution deletes
trailing newlines, so related test there cannot be changed
(and therefore could not notice this user experience flaw).