MM-10573 Add error page if user doesn't authorize Mattermost for OAuth (#9402)

api4/oauth.go

@@ -452,6 +452,15 @@ func completeOAuth(c *Context, w http.ResponseWriter, r *http.Request) {
 
 	service := c.Params.Service
 
+	oauthError := r.URL.Query().Get("error")
+	if oauthError == "access_denied" {
+		utils.RenderWebError(c.App.Config(), w, r, http.StatusTemporaryRedirect, url.Values{
+			"type":    []string{"oauth_access_denied"},
+			"service": []string{strings.Title(service)},
+		}, c.App.AsymmetricSigningKey())
+		return
+	}
+
 	code := r.URL.Query().Get("code")
 	if len(code) == 0 {
 		utils.RenderWebError(c.App.Config(), w, r, http.StatusTemporaryRedirect, url.Values{