Throw an exception if a template ID does not exist in your request body

[thinkingserious]

Issue Summary

You will not receive an error from the API is you send an email with the wrong template. Instead you will get a "Not Delivered" notice in your email activity feed. We want to make that experience better by throwing an exception in the code instead.

This issue is inspired by this.

Acceptance Criteria

• Use the API to verfiy a template ID exists, if not, throw an exception
• A new test is added that demonstrates that if you use a non-existent template, the code will throw an error

[Jerrgree]

I'll work on this