Send detailed error when user metadata are missing from redis

Include tip on how to restore the redis db from cartodb.

lib/cartodb/carto_data.js

@@ -27,14 +27,20 @@ module.exports = function() {
      * Get the database name for this particular subdomain/username
      *
      * @param req - standard express req object. importantly contains host information
-     * @param callback
+     * @param callback - gets called with args(err, dbname) 
      */
     me.getDatabase = function(req, callback) {
         // strip subdomain from header host
         var username = req.headers.host.split('.')[0]
         var redisKey = _.template(this.user_key, {username: username});
 
-        this.retrieve(this.user_metadata_db, redisKey, 'database_name', callback);
+        this.retrieve(this.user_metadata_db, redisKey, 'database_name', function(err, dbname) {
+          if ( err ) callback(err, null);
+          else if ( dbname === null ) {
+            callback(new Error("missing " + username + "'s dbname in redis (try CARTODB/script/restore_redis)"), null);
+          }
+          else callback(err, dbname);
+        });
     };
 
 
@@ -50,7 +56,13 @@ module.exports = function() {
         var username = req.headers.host.split('.')[0];
         var redisKey = _.template(this.user_key, {username: username});
 
-        this.retrieve(this.user_metadata_db, redisKey, 'id', callback);
+        this.retrieve(this.user_metadata_db, redisKey, 'id', function(err, dbname) {
+          if ( err ) callback(err, null);
+          else if ( dbname === null ) {
+            callback(new Error("missing " + username + "'s dbuser in redis (try CARTODB/script/restore_redis)"), null);
+          }
+          else callback(err, dbname);
+        });
     };
 
     /**
@@ -182,6 +194,8 @@ module.exports = function() {
     };
 
     // Redis Hash lookup
+    // @param callback will be invoked with args (err, reply)
+    //                 note that reply is null when the key is missing
     me.retrieve = function(db, redisKey, hashKey, callback) {
         this.redisCmd(db,'HGET',[redisKey, hashKey], callback);
     };