p9: Tflush should not result in an Rerror

[DeedleFake]

Currently, Tflush results in an Rerror due to a lack of support. While support would be nice at some point, the current behavior is a violation of the spec, which states that

A Tflush can never be responded to by an Rerror message.

In the interim before support is implemented, Tflush messages should simply be ignored and not responded to at all.