Add helpful error messages when importing optional dependencies

[theacodes]

Resolves #101

[dhermes]

1. How exhaustive were you in searching for places where this fails?
2. Can you add a tox environment without the deps instead of doing the pragma: NO COVER? Then you can integrate that into the coverage report?

[theacodes]

How exhaustive were you in searching for places where this fails?

Pretty exhaustive

Can you add a tox environment without the deps instead of doing the pragma: NO COVER? Then you can integrate that into the coverage report?

This'll make the coverage environment impossible to run standalone, unless I'm misunderstanding the request?

[dhermes]

This'll make the coverage environment impossible to run standalone, unless I'm misunderstanding the request?

That's correct, but the contents of this PR indicate that true coverage could not be done in a single environment.

[theacodes]

That's correct, but the contents of this PR indicate that true coverage could not be done in a single environment.

@dhermes that was already the case, we have several import guards.

[dhermes]

We can do better than sprinkling the NO COVER-s, so why not do better?

[theacodes]

@dhermes per our side conversion, created #127 to discuss this separately.

[dhermes]

SGTM LGTM