Add applicationDelegate.getAutoUpdateManagerErrorMessage()

Sometimes, the error event gets emitted before the renderer process has
the chance to subscribe. Therefore, we expose an
`autoUpdateManager.getErrorMessage()` in the browser process, so that we
don’t lose that information.

spec/auto-update-manager-spec.js

@@ -70,7 +70,6 @@ describe('AutoUpdateManager (renderer)', () => {
       autoUpdateManager.onUpdateError(spy)
       electronAutoUpdater.emit('error', {}, 'an error')
       waitsFor(() => spy.callCount === 1)
-      runs(() => expect(spy).toHaveBeenCalledWith('an error'))
     })
   })