Better check if authentication is wrong?

[godbout]

For v2:

Currently if the authentication is wrong, you can still prepare a timer and send it. Of course it will not be started. The list of recent timers, projects and tags will be empty. It's quite obvious that there's something wrong with authentication, and it doesn't break the workflow per se, but maybe add a better failed authentication response?

For example we could check for correct authentication when a timer service is getting enabled from the menu, and not enable it if authentication failed.