fix(client): show error if an adapter is removed

Before we would only show the error when this happens in the first run.
It's not likely to happen, but if the adapter is removed in a subsequent run, it would throw "null is not a function".

client/karma.js

@@ -170,6 +170,10 @@ var Karma = function(socket, iframe, opener, navigator, location) {
     }
   };
 
+  var UNIMPLEMENTED_START = function() {
+    this.error('You need to include some adapter that implements __karma__.start method!');
+  };
+
   // all files loaded, let's start the execution
   this.loaded = function() {
     // has error -> cancel
@@ -178,7 +182,7 @@ var Karma = function(socket, iframe, opener, navigator, location) {
     }
 
     // remove reference to child iframe
-    this.start = null;
+    this.start = UNIMPLEMENTED_START;
   };
 
   this.store = function(key, value) {
@@ -199,9 +203,7 @@ var Karma = function(socket, iframe, opener, navigator, location) {
 
   // supposed to be overriden by the context
   // TODO(vojta): support multiple callbacks (queue)
-  this.start = function() {
-    this.error('You need to include some adapter that implements __karma__.start method!');
-  };
+  this.start = UNIMPLEMENTED_START;
 
   socket.on('execute', function(cfg) {
     // reset hasError and reload the iframe