Abort when opening the RDB file results in an error other than ENOENT.

This fixes cases where the RDB file does exist but can't be accessed for
any reason. For instance, when the Redis process doesn't have enough
permissions on the file.

src/rdb.c

@@ -1069,7 +1069,6 @@ int rdbLoad(char *filename) {
 
     fp = fopen(filename,"r");
     if (!fp) {
-        errno = ENOENT;
         return REDIS_ERR;
     }
     rioInitWithFile(&rdb,fp);

src/redis.c

@@ -2654,7 +2654,7 @@ void loadDataFromDisk(void) {
             redisLog(REDIS_NOTICE,"DB loaded from disk: %.3f seconds",
                 (float)(ustime()-start)/1000000);
         } else if (errno != ENOENT) {
-            redisLog(REDIS_WARNING,"Fatal error loading the DB. Exiting.");
+            redisLog(REDIS_WARNING,"Fatal error loading the DB: %s. Exiting.",strerror(errno));
             exit(1);
         }
     }