Improve gRPC Errors example

The gRPC [Errors example](https://apple.github.io/servicetalk//servicetalk-examples/SNAPSHOT/grpc/index.html#errors) could be improved by adding to the server source:

- `try { } catch (Exception all) { return failed(mapToGrpcStatusException(all)); }` around endpoint.
- Add `GrpcExceptionMapperServiceFilter` to `GrpcServerBuilder` for mapping errors returned by other filters.

Start with the linked gRPC Errors example and locate its server source. Add the requested exception handling around the endpoint and configure GrpcExceptionMapperServiceFilter on GrpcServerBuilder; the example is complete when it demonstrates both error-mapping paths.

Written by the indexing model from the issue text.