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

@@ -127,6 +127,12 @@ func (gl *GitLab) Completions(prompt string) ([]deps.Dependency, error) {
 			continue
 		}
 
+		// 401 error needs special attention
+		// we instruct users to review the prerequisites of xray
+		if resp.StatusCode == http.StatusUnauthorized {
+			return nil, fmt.Errorf("Request failed (code 401). Please verify that the current project is in a root namespace or its descendant namespaces where the root namespace has a duo pro add-on. Refer to this page for more details: https://docs.gitlab.com/ee/user/project/repository/code_suggestions/repository_xray.html#enable-repository-x-ray")
+		}
+
 		if resp.StatusCode != http.StatusOK {
 			return nil, fmt.Errorf("Request failed (code %v)", resp.StatusCode)
 		}