Don't fail tfenv list in a virgin environment

[schlueter]

Addresses #187

tfenv list fails if no default has been set. This should not occur.

This PR this is addressed with these improvements:

Do not fail tfenv list because tfenv version name failed, and
don't display its errors.

When tfenv list is run and no default is set, helpfully note
that and offer a remedy.

[marcofranssen]

Had the same issue just today.

tfenv list will only work after you did a tfenv use. Would be great to have this fixed.

[marcofranssen]

Commenting here the following so the issue will automatically close if the PR gets merged.

resolves #187

[Zordrak]

closes #182

[Zordrak]

Thank you for this, I am adopting the approach, however:

Syntax doesn't quite follow standard.
Doesn't provide tests.
In constructing a test, this code fails the test as tfenv list will exit with code 1 when a default is set.

To resolve this I have create #236 based on your branch.