Fix bogus error message from merge-recursive error path

This error message should not usually trigger, but the function
make_cache_entry() called by add_cacheinfo() can return early
without calling into refresh_cache_entry() that sets cache_errno.

Also the error message had a wrong function name reported, and
it did not say anything about which path failed either.

Signed-off-by: Junio C Hamano <junkio@cox.net>

read-cache.c

@@ -24,7 +24,7 @@ unsigned int active_nr, active_alloc, active_cache_changed;
 
 struct cache_tree *active_cache_tree;
 
-int cache_errno;
+static int cache_errno;
 
 static void *cache_mmap;
 static size_t cache_mmap_size;