Clearer import errors when dependencies are not installed
At the moment you get an error message printed at the top and a stacktrace with a not so clear UnboundLocalError
Modify exception.args and reraise the exception for easier debugging when a python dependency is not installed.
Add _import_module_with_version_check function to share the import + version checks
Follow the same exception reraising pattern for gzip as for the other imports
Merge pull request #226 from lesteve/clearer-import-errors

Clearer import errors when dependencies are not installed