go.crypto/ssh: improve error message when no authentication methods remain
go.crypto/ssh: improve error message when no authentication methods remain

R=golang-dev
CC=golang-dev
http://codereview.appspot.com/5960044

Committer: Adam Langley <db68fba232b2c49ed3e9d3081df349dde7b61dfd@golang.org>

cmars/go.crypto,xman1101/terminal,polydawn-ports/go.crypto,tonnerre/golang-go.crypto,dizzyd/go.crypto,drhax9908/go.crypto,cmars/go.crypto,isislovecruft/go.crypto,cmars/sshlurp-go.crypto,dizzyd/go.crypto,xman1101/terminal,cmars/sshlurp-go.crypto,polydawn-ports/go.crypto,isislovecruft/go.crypto,drhax9908/go.crypto,tonnerre/golang-go.crypto