Generic response error handling

[Fank]

What type of PR is this?

• cleanup
• feature
• api-change

What this PR does / why we need it:

This PR replaces the existing request error handling with a more generic variation which allows it to extract the response body depending on the use case and handle the status code which isn't possible at the moment.

Which issue(s) this PR fixes:

Relates to #565 (comment)

Special notes for your reviewer:

I am not familiar with any kind of error messages, so far i only checked the insight API and there errors messages are not static or being able to parse to static structs.

Additional documentation e.g., usage docs, etc.:

The idea was to have use cases like:

issue, _, err := client.Issue.Get(context.Background(), "123", nil)
if err != nil && errors.Is(err, ErrNotFound) {
	// create new issue
} else err != nil {
	return err
}

issue, _, err := client.Issue.Create(context.Background(), &Issue{})
if err != nil && errors.Is(err, ErrValidation) {
	// check error content and proceed as preferred
} else err != nil {
	return err
}

[andygrunwald]

Thanks for this PR. I might take some time to review this, but it is still on my list.

One quick question:
Do you have any particular things in mind that need to be done before this PR can move out of the Draft state?