(Fix #215) better error printing in editor & extension, adds mapping for editor

extension/background.js

Index: extension/background.js
diff --git a/extension/background.js b/extension/background.js
index c1a6e5ff0b15b428f3e4d0b24a77526b790eaf51..9d85d7bf06af50e67d5b8cb159f0bd0c1a880439 100644
--- a/extension/background.js
+++ b/extension/background.js
@@ -27,6 +27,10 @@ function proxyUrls(port, requests) {
   port.postMessage({type: 'CREATE_DATA_URLS', requests: requests});
 }
 
+function proxyMessages(port, messages) {
+  port.postMessage({type: 'MESSAGES', messages: messages});
+}
+
 function onProxyUrls(requests) {
   var rulesToAdd = [];
   var ruleIdsToRemove = [];