We should error when local networking is specified with a sys mount type

[nfachan]

Use Cases

If I specify a job/test with local networking and a sys mount, it will fail eventually with an execution error. This is a little confusing. I would like it to be more explicit when it fails.

Acceptance Criteria

I get a better error message when I specify these incompatible settings. The message should point to documentation somehow. It doesn't have a to be a url, but maybe something like "search for X" or whatever.

Implementation Considerations

Definition of Done

• CHANGELOG.md updated.
• Documentation in doc/book/head updated.
• Issue is in the correct milestone.