New: add a custom error type for auth errors

client.go

@@ -131,6 +131,12 @@ func (c *Client) Do(ctx context.Context, req *http.Request, v interface{}) (*htt
 	// of the standard response. Others respond with a standardised error structure.
 	if c := resp.StatusCode; c >= 300 {
 
+		// Handle auth errors
+		if resp.StatusCode == http.StatusUnauthorized || resp.StatusCode == http.StatusForbidden {
+			err := AuthError(http.StatusText(resp.StatusCode))
+			return resp, err
+		}
+
 		// Try parsing the response using the standard error schema. If this fails we wrap the parsing
 		// error and return. Otherwise return the errors included in the API response payload.
 		var e = Errors{}

client_test.go

@@ -226,6 +226,30 @@ func TestDo(t *testing.T) {
 			tc.Error("should not return a response", cross)
 		}
 	})
+
+	t.Run("GET request that returns a forbidden response", func(tc *testing.T) {
+		client, mux, _, teardown := setup()
+		defer teardown()
+
+		mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+			checkMethod(tc, r, http.MethodGet)
+			w.WriteHeader(http.StatusForbidden)
+		})
+
+		req, _ := client.NewRequest("GET", ".", nil)
+		resp, err := client.Do(context.Background(), req, nil)
+
+		checkStatus(tc, resp, http.StatusForbidden)
+		checkHasError(tc, err)
+		if _, ok := err.(AuthError); ok == false {
+			t.Errorf("should return a starling.AuthError: %T", err)
+		}
+		if err, ok := err.(Error); ok == true && err.Temporary() == true {
+			t.Errorf("should not return a temporary error")
+		}
+
+	})
+
 }
 
 // Setup establishes a test Server that can be used to provide mock responses during testing.

errors.go

@@ -0,0 +1,15 @@
+package starling
+
+// Error specifies additional methods on the standard error interface
+type Error interface {
+	error
+	Temporary() bool
+}
+
+// AuthError indicates an issue with the authentication token
+type AuthError string
+
+func (e AuthError) Error() string { return string(e) }
+
+// Temporary indicates if an error is temporary
+func (e AuthError) Temporary() bool { return false }