sysroot: Add _require_booted_deployment() API

This is a common pattern that is replicated both in our code
and in rpm-ostree a lot.  Let's add a canonical API.

tests/admin-test.sh

@@ -95,7 +95,7 @@ echo "ok layout"
 if ${CMD_PREFIX} ostree admin deploy --stage --os=testos testos:testos/buildmaster/x86_64-runtime 2>err.txt; then
     fatal "staged when not booted"
 fi
-assert_file_has_content_literal err.txt "Cannot stage a deployment when not currently booted into an OSTree system"
+assert_file_has_content_literal err.txt "Cannot stage deployment: Not currently booted into an OSTree system"
 echo "ok staging does not work when not booted"
 
 orig_mtime=$(stat -c '%.Y' sysroot/ostree/deploy)