Incorrect access token results in an unintuitive error

[krassowski]

If user specifies an incorrect access token (for example wrapping the string value in quotes like token: "pat-randomstring" instead of token: pat-randomstring the error returned on attempt to download the repository will be:

File "nbgitpuller/pull.py", line 136, in resolve_default_branch
    raise ValueError(m)
ValueError: Problem accessing HEAD branch: https://repository.domain.git

This is not handled nicely upstream partially because nbgitpuller does not expect to be used with private repos.
Upstream issue:

• ValueError: Problem accessing HEAD branch jupyterhub/nbgitpuller#206