runtime: better error for send/recv on nil channel
runtime: better error for send/recv on nil channel

R=ken2
CC=golang-dev
http://codereview.appspot.com/1835041

d0f/go-zh,rflanagan/reginaldflanagan-project1,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,Triskite/willstone-goclone,Triskite/willstone-goclone,bryanxu/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,scirelli/scirelli-go,webfd/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,scirelli/scirelli-go,npe9/noahevans-go9,Triskite/willstone-goclone,rflanagan/reginaldflanagan-project1,Triskite/willstone-goclone,npe9/noahevans-go9,rdp/rogerpack2005-golang,mhennings/marcohennings-go,Triskite/willstone-goclone,sanjosh/sanjos100-tipc,bryanxu/go-zh,bryanxu/go-zh,npe9/noahevans-go9,mhennings/marcohennings-go,mhennings/marcohennings-go,npe9/noahevans-go9,Triskite/willstone-goclone,d0f/go-zh,d0f/go-zh,webfd/go-zh,mhennings/marcohennings-go,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,Triskite/willstone-goclone,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,rflanagan/reginaldflanagan-project1,mhennings/marcohennings-go,d0f/go-zh,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,sanjosh/sanjos100-tipc,bryanxu/go-zh,npe9/noahevans-go9,d0f/go-zh,scirelli/scirelli-go,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,d0f/go-zh,npe9/noahevans-go9,scirelli/scirelli-go,webfd/go-zh,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,bryanxu/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,scirelli/scirelli-go,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,Triskite/willstone-goclone,scirelli/scirelli-go,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,npe9/noahevans-go9,d0f/go-zh,npe9/noahevans-go9,rdp/rogerpack2005-golang