Stop using VoidCallback on WebAuth Logout

[lbalmaceda]

Changes

This VoidCallback usage is now replaced with a generic Callback<Void, AuthenticationException>. The "new" exception type has helpers to quickly check common error scenarios. e.g. browser app not available or user closed the browser. Besides the type change, the behavior of the class is the same.

The change is documented in the migration guide, and the tests have been updated.

Previously

onFailure(Auth0Exception error){
  if (error) {
    // logout failed, cause unknown.
  }
}

Now

onFailure(AuthenticationException error){
  if (error.isBrowserAppNotAvailable) {
    // Guide user to install a compatible browser app
  } else if (error.isAuthenticationCanceled) {
    // User closed the browser app and canceled the logout
  }
}