Do not cache non-success jsonp responses

Closes #186
Includes testcase

lib/cartodb/cartodb_windshaft.js

@@ -76,9 +76,14 @@ var CartodbWindshaft = function(serverOptions) {
       var that = this;
       var thatArgs = arguments;
       var statusCode;
-      if ( args.length > 2 ) statusCode = args[2];
-      else {
-        statusCode = args[1] || 200;
+      if ( res._windshaftStatusCode ) {
+        // Added by our override of sendError
+        statusCode = res._windshaftStatusCode;
+      } else {
+        if ( args.length > 2 ) statusCode = args[2];
+        else {
+          statusCode = args[1] || 200;
+        }
       }
       var req = res.req;
       Step (
@@ -117,6 +122,14 @@ var CartodbWindshaft = function(serverOptions) {
       );
     };
 
+    var ws_sendError = ws.sendError;
+    ws.sendError = function() {
+      var res = arguments[0];
+      var statusCode = arguments[2];
+      res._windshaftStatusCode = statusCode;
+      ws_sendError.apply(this, arguments);
+    };
+
     /**
      * Helper to allow access to the layer to be used in the maps infowindow popup.
      */