Return 0 when watchPosition() errors (#100)

Author: marcoscaceres

index.html

@@ -573,7 +573,7 @@ <h2>
               <li>[=Call back with error=] |errorCallback| and
               {{GeolocationPositionError/POSITION_UNAVAILABLE}}.
               </li>
-              <li>Return an [=implementation-defined=] {{long}}.
+              <li>Return 0.
               </li>
             </ol>
           </li>