Add method to check if auth is working

[longears]

We have hasRegistrationKeys() and hasPermanentKeys() which just check if the TentApp object has keys at all. Should have another method which checks if the keys work.

Call it verifyAuthKeys()

For example, this could be done by doing a getFollowers() and checking if the auth fails in the process. This will be easier once there is better error handling throughout python-tent-client.