Improve error msg when no deploy targets found. (#379)

lib/filterTargets.js

@@ -16,7 +16,9 @@ module.exports = function(options, validTargets) {
   }
 
   if (targets.length === 0) {
-    throw new FirebaseError('No targets found. Valid targets are: ' + validTargets.join(','), {exit: 1});
+    throw new FirebaseError('Cannot understand what targets to deploy. Check that you specified valid targets' +
+                            ' if you used the --only or --except flag. Otherwise, check your firebase.json to' +
+                            ' ensure that your project is initialized for the desired features.', {exit: 1});
   }
   return targets;
 };