Return proper error message if bin/omero import is called from OMERO.py

This commit wraps the classpath search using path.py and calls self.ctx.die()
is an OSError is thrown.
Add unit test for bin/omero import with client directory
Display OSError in the CLI die message as suggested by Simone

Also handle the case of existing directories with no JAR files
Add parametrized test to handle non-existing/empty client directory
Merge pull request #3080 from sbesson/omeropy_import_noclient

Add error message when importing with OMERO.py