fix(geolocation): retain Observable even during an error condition (#532)

The way this was setup previously, if an error occurred on the watchPosition observable, the observer was sent an error, which would have to be caught. This also has the side effect of completing the observable, which means anything down stream that would be subscribed would be unsubscribed and no longer receive updates.

Instead of using binding the error callback to ```observer.error``` this change just binds the error callback to ```observer.next``` and lets the subscriber filter out results that match ```PositionError``` rather than having to manage re-subscribing (which could just immediately fail and enter a loop of catch/retry)

src/plugins/geolocation.ts

@@ -62,6 +62,18 @@ export interface Geoposition {
   timestamp: number;
 }
 
+export interface PositionError {
+  /**
+   * A code that indicates the error that occurred
+   */
+  code: number;
+  
+  /**
+   * A message that can describe the error that occurred
+   */
+   message: string;
+}
+
 export interface GeolocationOptions {
   /**
    * Is a positive long value indicating the maximum age in milliseconds of a
@@ -140,7 +152,9 @@ export class Geolocation {
    * Observable changes.
    *
    * ```typescript
-   * var subscription = Geolocation.watchPosition().subscribe(position => {
+   * var subscription = Geolocation.watchPosition()
+   *                               .filter((p) => p.code === undefined) //Filter Out Errors
+   *                               .subscribe(position => {
    *   console.log(position.coords.longitude + ' ' + position.coords.latitude);
    * });
    *
@@ -151,10 +165,10 @@ export class Geolocation {
    * @param {GeolocationOptions} options  The [geolocation options](https://developer.mozilla.org/en-US/docs/Web/API/PositionOptions).
    * @return Returns an Observable that notifies with the [position](https://developer.mozilla.org/en-US/docs/Web/API/Position) of the device, or errors.
    */
-  static watchPosition(options?: GeolocationOptions): Observable<Geoposition> {
+  static watchPosition(options?: GeolocationOptions): Observable<Geoposition | PositionError> {
     return new Observable<Geoposition>(
       (observer: any) => {
-        let watchId = navigator.geolocation.watchPosition(observer.next.bind(observer), observer.error.bind(observer), options);
+        let watchId = navigator.geolocation.watchPosition(observer.next.bind(observer), observer.next.bind(observer), options);
         return () => navigator.geolocation.clearWatch(watchId);
       }
     );