Make error/warning notices consistent.
Minor refactorings.
Docstring changes, plus a fix to allow --help to work.
Fix the test to reflect moving the message from :out to :err.
Supports API changes to the Overlay project

These changes are required to account for a handful of changes made
while refactoring the Overlay project (see Overlay [pull request #3](https://github.com/immutant/overlay/pull/3) ).