Fix client http error handling and relative url root

Description

Previously, any error responses (i.e. 400) would still resolve the response. Now, these error responses are rejected. This is needed to appropriately handle errors while committing in the upcoming SourceControl MR. This is also needed by [Feat] Support empty projects (#53 - closed).

There's also more!

• We move web-ide-fs/.../utils/path into it's own package. It was necessary to reference these util functions in other packages.
• We add tests for GitLabClient which also fixes a bug with a GitLab URL that uses a relative URL root like (gitlab.example.com/gitlab)!

Screenshots

• With local running gdk under relative_url_root: '/gl'

Before	After