misleading error by wft4galaxy ... generate-test

[ilveroluca]

If one accidentally calls wft4galaxy with the subcommand generate-test the tools gives an error like

usage: wft4galaxy [-h] [--server GALAXY_URL] [--api-key GALAXY_API_KEY]
                  [-f FILE] [--enable-logger] [--debug] [--disable-cleanup]
                  [--output-format {text,xunit}] [--xunit-file FILE_PATH]
                  [-o PATH]
                  [test [test ...]]
wft4galaxy: error: Test file workflow-test-suite.yml doesn't exist or isn't a file

rather than saying that generate-test isn't a valid subcommand for the tool (it is used with wft4galaxy-wizard)