Add some basic tests to the mod portal client.

[Builditluc]

I don’t know if you already planned to add that but I would suggest adding tests for the different errors returned by the api

EDIT: I can also implement this if you want

[MForster]

Yes, that was my plan, see also #79. I'm trying to file issues and assign them to myself so that we can avoid working on the same thing at a time.

[Builditluc]

I’ve created an issue #83 that tries to list all of the testing cases we have to add so that we can mange who works on what and don’t work on the same thing