builder: handle anything we cannot parse the command for as a fatal error.

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
builder: provide a friendly message on parser errors

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
Merge pull request #8579 from erikh/builder_html_panic

builder: handle anything we cannot parse the command for as a fatal error