add missing `set -e`

ensuring that install.sh exits 1 if `go install` fails,
which ensures that make sees the failure

install.sh

 #!/bin/bash
 
+set -e
+
 if [ -z "$GOPATH" ]; then
 	echo \$GOPATH is unset: see https://golang.org/doc/code.html for details
 	exit 1