delete_refs(): make error message more generic

Change the error message from

    Could not remove branch %s

to

    could not remove reference %s

First of all, the old error message referred to "branch
refs/remotes/origin/foo", which was awkward even for the existing
caller. Normally we would refer to a reference like that as either
"remote-tracking branch origin/foo" or "reference
refs/remotes/origin/foo". Here I take the lazier alternative.

Moreover, now that this function is part of the refs API, it might be
called for refs that are neither branches nor remote-tracking
branches.

While we're at it, convert the error message to lower case, as per our
usual convention.
Signed-off-by: NMichael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

refs.c

@@ -2843,7 +2843,7 @@ int delete_refs(struct string_list *refnames)
 		const char *refname = refnames->items[i].string;
 
 		if (delete_ref(refname, NULL, 0))
-			result |= error(_("Could not remove branch %s"), refname);
+			result |= error(_("could not remove reference %s"), refname);
 	}
 
 	return result;