shellDBus: Fix error message returned from Eval

Annoyingly, `message` on errors are non-enumerable, which mean that
JSON.stringify on the error will produce `{}`. Just cast it to a string
for now.

js/ui/shellDBus.js

@@ -119,7 +119,7 @@ const GnomeShell = new Lang.Class({
                 returnValue = '';
             success = true;
         } catch (e) {
-            returnValue = JSON.stringify(e);
+            returnValue = '' + e;
             success = false;
         }
         return [success, returnValue];