go.crypto/ssh: avoid recover() when handling invalid channel ids
go.crypto/ssh: avoid recover() when handling invalid channel ids

This proposal removes the use of recover() to catch
invalid channel ids sent from the remote side. The
recover() unfortuntaly makes debugging harder as it
obscures other panic causes.

Another source of panic()s exists inside marshal.go,
which will be handled with in a later CL.

R=agl, gustav.paul
CC=golang-dev
https://golang.org/cl/6404046

lucevers/crypto,pki-io/crypto,wooutl/golangcrypto,juniorz/crypto,tca/crypto,zj8487/crypto,conseweb/golangcrypto,thomaso-mirodin/crypto,cmars/tools,wooutl/golangcrypto,Carrotman42/crypto,progrium/crypto,cheffo/crypto,ScriptRock/crypto,golang/crypto,cmars/tools,CodyGuo/crypto,zj8487/crypto,progrium/crypto,a-palchikov/crypto,conseweb/golangcrypto,golang/crypto,Carrotman42/crypto,juniorz/crypto,ScriptRock/crypto,guaiguaihw/crypto,lucevers/crypto,kim-racktop/crypto,thomaso-mirodin/crypto,kim-racktop/crypto,cheffo/crypto,a-palchikov/crypto,guaiguaihw/crypto,golang/crypto,mattharden/crypto,bford/golang-x-crypto,cmars/tools,bford/golang-x-crypto,mattharden/crypto,tca/crypto,CodyGuo/crypto,pki-io/crypto,cmars/tools