Return non-zero exit code if unzip fails
shutil.rmtree only works on directories, use os.unlink for files.
Check http code when requesting XML (avoids misleading parse error)
Replace sys.exit with raise Stop
Replace print with log.{debug,info,warn,error}
Merge pull request #11 from manics/minor_fixes

Minor issues found whilst setting up OMERO-OMEGO-upgrade-example