Make the 'lock file' exists error more informative

It looks like someone did 90% of the work, then forgot to actually use
the function in one place.

Also the helper function did not use the correct variable.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

lockfile.c

@@ -158,7 +158,7 @@ static int lock_file(struct lock_file *lk, const char *path, int flags)
 
 NORETURN void unable_to_lock_index_die(const char *path, int err)
 {
-	if (errno == EEXIST) {
+	if (err == EEXIST) {
 		die("Unable to create '%s.lock': %s.\n\n"
 		    "If no other git process is currently running, this probably means a\n"
 		    "git process crashed in this repository earlier. Make sure no other git\n"
@@ -184,7 +184,7 @@ int hold_lock_file_for_append(struct lock_file *lk, const char *path, int flags)
 	fd = lock_file(lk, path, flags);
 	if (fd < 0) {
 		if (flags & LOCK_DIE_ON_ERROR)
-			die("unable to create '%s.lock': %s", path, strerror(errno));
+			unable_to_lock_index_die(path, errno);
 		return fd;
 	}