Merge pull request #4249 from shykes/builtins

Register built-in engine commands at runtime instead of compile-time
Engine: clearer job error reporting

* Job.Error and Job.Errorf always print a newline.
* Avoid redundant prefixing of error messages.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)