sudo.bash: diagnose when the go tool is not in $PATH

Fixes #4386.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/6854050

src/sudo.bash

@@ -12,6 +12,12 @@ Darwin)
 	exit 0
 esac
 
+# Check that the go command exists
+if ! go help >/dev/null 2>&1; then
+	echo "The go command is not in your PATH." >&2
+	exit 2
+fi
+
 eval $(go env)
 if ! [ -x $GOTOOLDIR/cov -a -x $GOTOOLDIR/prof ]; then
 	echo "You don't need to run sudo.bash." >&2