Certification test should excercise errors

[helmutg]

The test_certification.py test only tests valid interactions. It does not validate that errors are correctly communicated. For instance, passing invalid arguments should likely result in an org.varlink.service.InvalidParameter error, but an implementation that would close the connection upon encountering any error would pass certification as the interaction does not provoke any errors.

Is the org.varlink.certification.varlink interface set in stone or would it be acceptable to extend it such that an error needs to be provoked?

[behrmann]

I agree that the certification test should probably also check error cases. I'll have a look at how the other implementations handle this.