rerere: unify error messages when read_cache fails

We have multiple different variants of the error message we show to
the user if 'read_cache' fails.  The "Could not read index" variant we
are using in 'rerere.c' is currently not used anywhere in translated
form.

As a subsequent commit will mark all output that comes from 'rerere.c'
for translation, make the life of the translators a little bit easier
by using a string that is used elsewhere, and marked for translation
there, and thus most likely already translated.

"index file corrupt" seems to be the most common error message we show
when 'read_cache' fails, so use that here as well.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

rerere.c

@@ -568,7 +568,7 @@ static int find_conflict(struct string_list *conflict)
 {
 	int i;
 	if (read_cache() < 0)
-		return error("Could not read index");
+		return error("index file corrupt");
 
 	for (i = 0; i < active_nr;) {
 		int conflict_type;
@@ -601,7 +601,7 @@ int rerere_remaining(struct string_list *merge_rr)
 	if (setup_rerere(merge_rr, RERERE_READONLY))
 		return 0;
 	if (read_cache() < 0)
-		return error("Could not read index");
+		return error("index file corrupt");
 
 	for (i = 0; i < active_nr;) {
 		int conflict_type;
@@ -1103,7 +1103,7 @@ int rerere_forget(struct pathspec *pathspec)
 	struct string_list merge_rr = STRING_LIST_INIT_DUP;
 
 	if (read_cache() < 0)
-		return error("Could not read index");
+		return error("index file corrupt");
 
 	fd = setup_rerere(&merge_rr, RERERE_NOAUTOUPDATE);
 	if (fd < 0)