WIP: Nonexisting exercise submission

Not really done with the PR yet, but wanted to put it here to get some feedback. I encountered issue similar to https://github.com/exercism/exercism.io/issues/1520, so I wanted the feedback message to include more details.

Two things that would help me finish this:

* In the issue, @kytrinyx mentioned that _"Perhaps we simply need to ask the API whether or not the exercise exists when the exercise gets submitted"_ so I wanted a pointer to where a list of all available exercises is stored. It seems like currently the list of exercises is generated from available files, so the best I could think of is make sure the language / extension is recognized.

* Is there a good way to test out CLI with local website (like specify a local endpoint)?