Check whether the probe for code repositories can return more specifc errors

[lorenzo-cavazzi]

The probe GET /repositories/<id> returns 500 errors when the repository isn't accessible. Sometimes that requires only re-connecting (E.G: GitHub connection seems to expire after a while and re-connecting fixes it).

More info here https://www.notion.so/renku/Help-users-resolve-Git-connection-error-1a40df2efafc80eaa110ef43c235421e

Perhaps there is a way to identify these problems and return more specific errors?