Bug 721156 - aError.stack is undefined in the Error Console when exception is thrown in Scratchpad. r=robcee

browser/devtools/scratchpad/scratchpad.js

@@ -487,7 +487,8 @@ var Scratchpad = {
    */
   writeAsErrorComment: function SP_writeAsErrorComment(aError)
   {
-    let newComment = "Exception: " + aError.message + "\n" + aError.stack.substring(0, aError.stack.length - 1);
+    let stack = aError.stack || aError.fileName + ":" + aError.lineNumber;
+    let newComment = "Exception: " + aError.message + "\n" + stack.replace(/\n$/, "");
     
     this.writeAsComment(newComment);
   },