Add error message when 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:
- check the new CLI unit test passes.
- download `OMERO.py` from the merge build
  -  run `bin/omero import` and check the error message makes sense `(No such file or directory)`
  -  create an empty `lib/client` folder, run `bin/omero import` and check the error message makes sense


Is there a better way to handle this exception, e.g. at the `path.files()` level? /cc @simleo