Use error.stack instead of error for console log

Chrome/V8 give you the error name, message and callstack at that
property, where toString() gives you [object NavigatorUserMediaError]

FREEBIE

js/background.js

@@ -156,7 +156,7 @@
         });
         var error;
         if ((error = c.validateNumber())) {
-          console.log(error);
+          console.log(error.stack);
           return;
         }