Allow initializing workflows from failed invocations

[bernt-matthias]

I wanted to debug a failed invocation and wanted to download everything for local testing. Like so

planemo workflow_test_init --from_invocation <your_invocation_id>  --galaxy_url <your_prefered_galaxy_instance_url> --galaxy_user_key <your_api_key>

This failed when downloading the failed test data (with some python stack trace). Would be nice if planemo would still download everything (and create the test yaml) and fail afterwards. This behavior might also be triggered with a CLI flag.