Improve error handling

[erikbosch]

This intends to give better error handling for velocitas init and velocitas sync.

Background is that you sometimes got misleading errors, like:

• If the repo specified in velocitas.json did not exist it as reported as "format error"
• For some git errors "raw" simple-git expections were shown, now I try to give more context specific velocitas errors. If you want the simple-git ones you need to use verbose mode
• I try to do some more tests after clone. Hopefully to not just silently ignore that we failed to clone due to rate limit.

Below is an example of how it could look like after this PR when trying to use a non-existing repo

I did some updates to testing to make things running, but in general the mocking is quite messy, especially if you want to trigger specific error situations.

[erikbosch]

FYI @mikehaller - this one may give better errors if clone fails due to rate limiting.