Send Insight emails as HTML via Mandrill

Retains old behavior for ThinkUp installs without Mandrill configured.

Closes #1779, closes #1781

Handle missing template for mailHTMLViaMandrill

Explicitly catch Mandrill_Unknown_Template and rethrow when sending.
In InsightsGenerator email process, revert to non-templated mail in this case.
Throw other errors as was currently handled.
Add support in the isTest() mode to triggere exceptions.

Added Insights Generator Configuration to Docs