decode: update API to return error

decode: update API to return error

In some cases, the decoding is not possible and some really invalid
packet can be created. This is in particular the case of tunnel. In
that case, it is more interesting to forget about the tunneled
packet and only consider the original packet.

DecodeTunnel function is maked as warn_unused_result because it is
meaningful for the decoder to know if the underlying data were not
correct. And in this case, only focus detection on the content.