lualoader: drop the filename and word "LUA" from errors

The filename is nearly always wrong since it's /boot/lua/loader.lua, which
is not useful for diagnostics. The actual errmsg will include a lua filename
if this is relevant.

Dropping "LUA" while we're here because that's almost universally
irrelevant to whatever error follows, unless the error states that it's
actually a lua problem.

Both of these are minor nits that just detract from identifying the
pertinent information.

MFC after: 3 days