(TK-71/74) Improve Webrouting error messages

Improve error messages in the Webrouting service when
a specified service or endpoint does not appear in the
webrouting configuration and when the :web-router-service
section of the TK configuration is absent.

This PR addresses both TK-71 and TK-74.
I don't think `IllegalStateException` is appropriate here.  IIRC we tend to use `IllegalArgumentException` in this situation.

Also, as a longer-term improvement, it might be worth defining something like `puppetlabs.trapperkeeper.InvalidConfigurationException` that we could use in situations like this, since it's something that comes up all over the place, and `IllegalArgumentException` has a very specific semantic that also doesn't really apply here.