Add prerequisite for this 401 errors

What does this MR do and why?

Closes #32 (closed)

Extend the error message with more details explaining to double check Duo Pro subscription and permissions for a user. Additionally, provide a link to the documentation page.

internal/aiclient/gitlab.go

@@ -55,6 +55,10 @@ type GitLabResponse struct {
 	Completion string `json:"response"`
 }
 
+type GitLabErrorResponse struct {
+	Message string `json:"message"`
+}
+
 func NewGitLab(scannerVersion, apiV4Url, jobID, token string) *GitLab {
 	return &GitLab{
 		ApiV4Url:       apiV4Url,
@@ -128,7 +132,13 @@ func (gl *GitLab) Completions(prompt string) ([]deps.Dependency, error) {
 		}
 
 		if resp.StatusCode != http.StatusOK {
-			return nil, fmt.Errorf("Request failed (code %v)", resp.StatusCode)
+			var gler GitLabErrorResponse
+			err = json.NewDecoder(resp.Body).Decode(&gler)
+			if err != nil {
+				return nil, fmt.Errorf("Failed to decode response body: %v", err)
+			}
+
+			return nil, fmt.Errorf(gler.ErrorMessage(resp.StatusCode))
 		}
 
 		break
@@ -147,3 +157,11 @@ func (gl *GitLab) Completions(prompt string) ([]deps.Dependency, error) {
 func (glr *GitLabResponse) String() string {
 	return strings.TrimSuffix(glr.Completion, prompt.ResponseClosingTag)
 }
+
+func (gler *GitLabErrorResponse) ErrorMessage(statusCode int) string {
+	if gler.Message != "" {
+		return fmt.Sprintf("Request failed (code %d): %s", statusCode, gler.Message)
+	}
+
+	return fmt.Sprintf("Request failed (code %d)", statusCode)
+}