lualoader: Improve module loading diagnostics

Some fixes:

• Maintain historical behavior more accurately w.r.t verbose_loading; verbose_loading strictly prints "${module_name...}" and later "failed!" or "ok" based on load success
• With or without verbose_loading, dump command_errbuf on load failure. This usually happens prior to ok/failed if we're verbose_loading

Reviewed by: imp
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D17694