`http_error_or_content()` should print explicit error messages when the gitlab API raises an error

[ymansiaux]

Validation

• When the user provides an incorrect token an appropriate error message is shown

• When the user tries to access a repository which doesnt exist an appropriate error message is shown

• When the user tries to access a repository which is private an appropriate error message is shown

Dev - Tech

Errors to deal with :

• "404 Project Not Found" : the repository does not exist or is private

• "401 Unauthorized": the token is missing or incorrect

• "403 Forbidden" : shown (notably) when the user does not have the right "Role" (we need to ensure that this type of error can not be seen elsewhere