Gracefully handle unknown host exceptions during OpenID discovery

We don't need these errors in our application server log.  If the
client has given us a hostname we can't resolve, trap the error
and return an invalid discovery JSON payload to them instead.  The
user will see a message explaining its an unsupported/invalid URL
and can try again.

Signed-off-by: Shawn O. Pearce <sop@google.com>