cmd/go: don't add detail to errPrintedOutput
cmd/go: don't add detail to errPrintedOutput

This makes the last error-reporting CL a bit less
aggressive.  errPrintedOutput is a sentinel value
that should not be wrapped.

R=gri
CC=golang-dev
http://codereview.appspot.com/5845052

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang