config: move check for no configured method
config: move check for no configured method

First check all error conditions before getting to the warnings (this is
important for --generate-key).

Also, downgrade the exit_bug to exit_error if method `null' is not supported.

fpletz/fastd,rlei/fastd-android,fpletz/fastd,rlei/fastd-android,rlei/fastd-android,fpletz/fastd,fpletz/fastd,rlei/fastd-android,rlei/fastd-android,fpletz/fastd