500 error when logging in with non-resolvable domains

[martymcguire]

Chat log: https://chat.indieweb.org/dev/2017-03-21#t1490113485026000

aaronpk attempted to log into screech using a local .dev site and it returned a 500 error.

likely, flask-micropub attempted to find the auth and token endpoints and couldn't resolve them, leading to a 500. these errors should be caught and a useful error message given.