service: fix memory leak introduced in parse_challenge()

Make sure that failure_reason is free'd by refactoring the code so
that the parent function handles allocation and parse_challenge only
parses the challenge, not the whole error message.

https://mail.gnome.org/archives/networkmanager-list/2017-April/msg00007.html

[thaller@redhat.com: fix whitespace and idiomatic commit message subject]

Fixes: 5b96fecb