go.crypto/ssh/agent: prefix errors with "agent".
go.crypto/ssh/agent: prefix errors with "agent".

R=agl, dave, jpsugar
CC=golang-codereviews
https://codereview.appspot.com/87810047

Committer: Adam Langley <db68fba232b2c49ed3e9d3081df349dde7b61dfd@golang.org>

isislovecruft/go.crypto,polydawn-ports/go.crypto,isislovecruft/go.crypto,polydawn-ports/go.crypto