(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'm also +1 on this. Should we open a Jira to create this exception and change the various TK services to use it?