Do not include exception message for unresolved exceptions #352
Description
In addition to the exception logging improvements for #336, ExceptionResolversExceptionHandler should not use the exception message, and thus leak implementation details.
By default, an unresolved exception will be logged, but otherwise the client will receive just a generic message like "Internal error". It would be up applications to have one or more DataFetcherExceptionResolver components to handle specific exceptions, or otherwise use a generic message with additional information about the request, like the execution id for example.