Feat/decode gitlab error

[puffitos]

Motivation

Relates to #231

Changes

Added a general error struct, in which we can decode API error messages and the non-OK status, to return as an error. Additionally, I've added contexts to the target manager's logging calls.

Tests done

Recreated a case which can create this POST error, by creating a file which has the same DNS name as my dev sparrow b
but a different URL scheme (http vs https). Getting the following logs now (yes, I know we have duplicate logs, but this shouldn't be addressed in this PR):

time=19:26:03 level=ERROR source=/Users/puffito/dev/repos/sparrow/pkg/sparrow/targets/remote/gitlab/gitlab.go:321 msg="Failed to post file" status="400 Bad Request"
time=19:26:03 level=ERROR source=/Users/puffito/dev/repos/sparrow/pkg/sparrow/targets/manager.go:207 msg="Failed to register global gitlabTargetManager" error="gitlab API sent an unexpected status code (400) with the following error message: {\"message\":\"A file with this name already exists\"}"
time=19:26:03 level=WARN source=/Users/puffito/dev/repos/sparrow/pkg/sparrow/targets/manager.go:127 msg="Failed to register self as global target" error="gitlab API sent an unexpected status code (400) with the following error message: {\"message\":\"A file with this name already exists\"}"

• Unit tests succeeded
• E2E tests succeeded

TODO

• I've assigned this PR to myself
• I've labeled this PR correctly

[niklastreml]

LGTM, just wondering why you're not exposing the apiError type and chose to keep it private

[puffitos]

LGTM, just wondering why you're not exposing the apiError type and chose to keep it private

Not needed anywhere else and it's extremely specific to the gitlab specification. Looks like a pretty solid candidate for an unexposed type. 😊