Go through zrpc and add error context to every function that can create
error. This is similar to what NEO does in neonet. The reason to do this
was the following obscure error from wcfs:

    E1105 11:32:33.295497   24639 wcfs.go:2505] zwatch zeo://:28359: EOF

which, after this patch, is now

    E1105 12:53:15.922052   30731 wcfs.go:2510] zwatch zeo://:27024: zlink 127.0.0.1:47768 - 127.0.0.1:27024: recvPkt: EOF