fix(client): handle error responses with detailed messages

Adds error handling for non-OK HTTP status codes in the client.
Implements custom error messages by reading the response body
when the status code indicates an error, ensuring better
debugging and clarity during failures.
Enhances unit tests to cover unauthorized access and incorrect 
body length scenarios, validating the error handling mechanism.

gitlab/client.go

@@ -69,6 +69,12 @@ func (r *RestClient) projectApiCall(method, project string, api string, body io.
 	if resp.StatusCode == http.StatusOK && response != nil {
 		decoder := json.NewDecoder(resp.Body)
 		err = decoder.Decode(response)
+	} else if resp.StatusCode != http.StatusOK {
+		buff, err2 := io.ReadAll(resp.Body)
+		if err2 != nil {
+			return fmt.Errorf("error reading body: %w", err2)
+		}
+		return fmt.Errorf("status %d: %s", resp.StatusCode, string(buff))
 	}
 	return err
 }