submodule: add newline on invalid submodule error

Since 'err' contains output for multiple submodules and is printed all
at once by fetch_populated_submodules(), errors for each submodule
should be newline separated for readability. The same strbuf is added to
with a newline in the other half of the conditional where this error is
detected, so make the two consistent.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

submodule.c

@@ -1478,7 +1478,7 @@ static int get_next_submodule(struct child_process *cp,
 			    !is_empty_dir(ce->name)) {
 				spf->result = 1;
 				strbuf_addf(err,
-					    _("Could not access submodule '%s'"),
+					    _("Could not access submodule '%s'\n"),
 					    ce->name);
 			}
 		}