Fix FinalErrorHandler for error phase

[groyoh]

Expected behavior

When using a Layer, an unrecovered panic should cause the FinalErrorHandler to be use and return a 500 status code with the error message.

Actual behavior

The FinalErrorHandler was never used if a panic occured. The Run function simply returned without calling serveHTTP (ref. https://github.com/vinci-proxy/layer/blob/master/layer.go#L143) and therefore simply swallows the panic. @h2non correct me if this was the expected behavior.

Additional information

This PR should also increase code coverage.

[h2non]

You are right. I think we should include a error specific final handler function to be custimized, istead of using finalHandler. Currently it's bad vecause we are using the same handler for any phase.