Add response code to errors

[philipithomas]

Debugging clearbit errors in our code has become a nightmare. This is what it looks like in our logs:

ERROR: clearbit:

Manually debugging, I've realized that this is because the API is returning codes like 404 or 422.

Internally, we end up wrapping the errors with github.com/pkg/errors and having to return different types of errors:

	res, resp, err := s.ClearbitClient.Person.FindCombined(clearbit.PersonFindParams{
		Email: r.Email,
	})

	if resp.StatusCode == 202 {
		// Deferred
		return r, errTryAgainLater
	} else if resp.StatusCode == 404 {
		return r, errEmailNotFound
	} else if resp.StatusCode == 422 {
		return r, errInvalid
	} else if err != nil {
		return r, errors.Wrap(err, fmt.Sprintf("code %d", r.StatusCode))
	}

I propose that this package either (1) has named error types that it returns (e.g. ErrNotFound), or that it at least wraps all errs with the API response status code.