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

stand/common/interp_lua.c

@@ -126,7 +126,7 @@ interp_init(void)
 	if (interp_include(filename) != 0) {
 		const char *errstr = lua_tostring(luap, -1);
 		errstr = errstr == NULL ? "unknown" : errstr;
-		printf("Startup error in %s:\nLUA ERROR: %s.\n", filename, errstr);
+		printf("ERROR: %s.\n", errstr);
 		lua_pop(luap, 1);
 		setenv("autoboot_delay", "NO", 1);
 	}