Provide an error code for "User did not consent to login" error

<!-- ⚠️⚠️ Do Not Delete This! feature_request_template ⚠️⚠️ --> <!-- Please read our Rules of Conduct: https://opensource.microsoft.com/codeofconduct/ --> <!-- Please search existing issues to avoid creating duplicates. --> <!-- Describe the feature you'd like. --> It would be useful to be able to detect the "User did not consent to login" error thrown by `authentication.getSession` when a user doesn't authorize github through VS Code. Right now the error just returns a string, so the only way to verify it's this error is by comparing the strings which won't work if the string changes or for other languages.