Bug #680961 - Broken dispatching of EAuthenticationSession

There was basically no error reporting from the server to the client
when any such occurred during authentication on the server side, thus
the client was left waiting for a response which never happened. Even
the 'dismiss' signal has a comment that it can be used for such cases,
it's not the right thing to do, because it hides the errors from a user,
thus he/she doesn't know that something went wrong. For that a new
ServerError signal was added and the error is properly propagated to
the client. Note the GOA errors don't strip the GDBus error from
the message, which looks odd in the UI, thus I added a workaround
for that too.