Handle the silly exception thrown from options.resetPref.

common/content/options.js

@@ -985,8 +985,15 @@ function Options() //{{{
         },
 
         resetPref: function (name)
+        {
+            try
             {
                 return service["pref"].clearUserPref(name);
+            }
+            catch (e)
+            {
+                // ignore - thrown if not a user set value
+            }
         },
 
         // this works only for booleans