Revert "loader/lua: Remove workaround for command_error"

This reverts commit 552f3072af.

loader.command_error was added just after 11.2, but appears to not have
been back ported to 11.x. 11.0 was the first lua loader release, so keep
this compat shim until we sort out what to do.

MFC After:		3 days
Sponsored by:		Netflix
Reviewed by:		kevans
Differential Revision:	https://reviews.freebsd.org/D45883

stand/lua/config.lua

@@ -407,7 +407,12 @@ local function loadModule(mod, silent)
 			end
 
 			if cli_execute_unparsed(str) ~= 0 then
-				print(loader.command_error())
+				-- XXX Temporary shim: don't break the boot if
+				-- loader hadn't been recompiled with this
+				-- function exposed.
+				if loader.command_error then
+					print(loader.command_error())
+				end
 				if not silent then
 					print("failed!")
 				end