don't suppress failures in command modules

When something is wrong in apkg command module, apkg responds with:

Invalid apkg command: foo

Even though the command is available, just broken.

This was introduced by py3.5 compatibility because ModuleNotFoundError doesn't exist on py3.5 yet and thus there is no way do determine if module is missing of failed to load from the generic ImportError.

However, this is really bad way to communicate an error, apkg needs to be fixed to show real reason for failure.