Put respective error message in exception.
Log DNS mismatch with critical severity.

Otherwise, users might miss the message if they are only interested in critical
log messages.
Update domains for DNS resolution.

- Add IPv4 addresses for <www.wikileaks.org>.
- Add <www.i2p2.de>.
Catch DNS resolution errors.

The exit relay might be unable to resolve the domain for some reason.  In that
case, we should catch and display the respective exception.