Error handling in TwistedResolver
Add some markup to the old template intro docs.

Closes #967.
Merge pull request #965 from ajdavis/twisted-resolver-err

Error handling in TwistedResolver
Disable the "bad_host" tests on CaresResolver

This is an unfortunate hack, but c-ares doesn't handle fake NXDOMAIN
responses in the same way as other resolvers.
Mark another timing-sensitive test as skipOnTravis.