Raise SubprocessError for non-zero returncode of the ERT subprocess

[wouterjdb]

This fix makes sure that when ERT fails, for whatever reason, a SubprocessError is raised and mlflow registers the run as a failed run.

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Contributor checklist

• [x] :tada: This PR closes #269.
• [x] :scroll: I have broken down my PR into the following tasks:
  • [x] Add raise when returncode > 0
• [ ] :robot: I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR.
• [x] :book: I have considered adding a new entry in CHANGELOG.md.
• [x] :books: I have considered updating the documentation.