an implementation is missing

```
play.api.http.HttpErrorHandlerExceptions$$anon$1: Execution exception[[RuntimeException: scala.NotImplementedError: an implementation is missing]]
at play.api.http.HttpErrorHandlerExceptions$.throwableToUsefulException(HttpErrorHandler.scala:293)
at play.api.http.DefaultHttpErrorHandler.onServerError(HttpErrorHandler.scala:220)
at play.api.GlobalSettings$class.onError(GlobalSettings.scala:160)
at play.api.DefaultGlobal$.onError(GlobalSettings.scala:188)
at play.api.http.GlobalSettingsHttpErrorHandler.onServerError(HttpErrorHandler.scala:100)
at play.core.server.netty.PlayRequestHandler$$anonfun$2$$anonfun$apply$1.applyOrElse(PlayRequestHandler.scala:100)
at play.core.server.netty.PlayRequestHandler$$anonfun$2$$anonfun$apply$1.applyOrElse(PlayRequestHandler.scala:99)
at scala.concurrent.Future$$anonfun$recoverWith$1.apply(Future.scala:344)
at scala.concurrent.Future$$anonfun$recoverWith$1.apply(Future.scala:343)
at scala.concurrent.impl.CallbackRunnable.run(Promise.scala:32)
```

Start by tracing the exception from play.api.http.HttpErrorHandlerExceptions in HttpErrorHandler.scala, through DefaultHttpErrorHandler and GlobalSettings, using the provided stack trace as the entry point. Identify where the NotImplementedError originates in play-googleauth and determine the expected behavior before defining a testable fix.

Written by the indexing model from the issue text.