Import failures prevent from re-trying

[etanol]

During development, it is common to import faulty modules from the repl. If a Hy module fails to load due to a HyTypeError or LexException, it cannot be loaded again until the repl is restarted.

There is a different exception treatment in the importer. One of the exception blocks is not cleaning up sys.modules before returning and that leaves a trace of the failure.

[etanol]

The root cause of this is somehow also related to the root cause of #1134. However, the fix is different.

[vodik]

If you're interested, dig into #1514 and #1518. I started digging into this a few weeks ago and there's a lot of problems with the loader.

[gilch]

Probably related to #712.

As I mentioned in #1514, "I think the corrupted module object remains in the module cache. We're supposed to remove it if it doesn't load."

[etanol]

I think efforts should go into completing #1518. Hy's importer is not even compliant with PEP-0302, therefore it's a matter of implementing it right in order to match the expectations of the average Python user.

[brandonwillard]

This should be closed by #1672 (and/or after #1683 for Python 2.7).

[etanol]

Definitely.