Incorporate feedback from review:

* Don't return anything from sendmail(); the function is used for side
  effects only.
* Import exceptions from smtplib so that users of the code don't have to
  search for them.

I believe there is later stuff to build - a KruxMailer object that does
stats/logging/retries and DTRT with exceptions. But that's for a later
release, right now all I need is a simple mailing function.