Better error handling for scp uploads

[andreasknoepfle]

In the context of #160 we debugged a bit into certain cases, when we have files that do not exists or when we have no permission. We have a very first draft in handling those cases.

Probably this is also liked to #62.

Description

We added some clauses that treat output on stderr or warnings that we do not recognize to be fatal and lead to error output.

Motivation and Context

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes (with unit and/or functional tests).
• [ ] I have added a note to CHANGELOG.md if necessary (in the ## master section).

[pmeinhardt]

This needs rebasing once #166 is merged so we get CI feedback on this. ✌️

[andreasknoepfle]

Rebased, but there seems to be an issue with the CI.