Add error message while importing with OMERO.py

Reported by @manics during the testing of 5.1.0-m1, this PR wraps the classpath search of `bin/omero import` and return a meaningful error message if the command is run from OMERO.py.

To test this PR;
- downloads `OMERO.py`, run `bin/omero import` and check the error message makes sense
- check the additional CLI unit test passes.

Is there a better way to handle this exception, e.g. at the `path.files()` level? /cc @simleo