pkg/lua: Make the module searchers conform to the API.
pkg/lua: Make the module searchers conform to the API.

The module searchers in the `require` package should return a string
if the module is not found, and not raise an error.

See: https://www.lua.org/manual/5.3/manual.html#pdf-package.searchers

Also make error strings contain newlines and tabs just like the original
ones.

miri64/RIOT,yogo1212/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,basilfx/RIOT,basilfx/RIOT,toonst/RIOT,toonst/RIOT,authmillenon/RIOT,aeneby/RIOT,mfrey/RIOT,authmillenon/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,cladmi/RIOT,OlegHahm/RIOT,kYc0o/RIOT,ant9000/RIOT,smlng/RIOT,yogo1212/RIOT,authmillenon/RIOT,x3ro/RIOT,jasonatran/RIOT,mfrey/RIOT,OTAkeys/RIOT,jasonatran/RIOT,mtausig/RIOT,mfrey/RIOT,smlng/RIOT,basilfx/RIOT,OTAkeys/RIOT,aeneby/RIOT,mfrey/RIOT,basilfx/RIOT,rfuentess/RIOT,miri64/RIOT,smlng/RIOT,toonst/RIOT,josephnoir/RIOT,josephnoir/RIOT,mtausig/RIOT,kaspar030/RIOT,josephnoir/RIOT,ant9000/RIOT,authmillenon/RIOT,mtausig/RIOT,yogo1212/RIOT,yogo1212/RIOT,rfuentess/RIOT,mtausig/RIOT,miri64/RIOT,x3ro/RIOT,jasonatran/RIOT,smlng/RIOT,aeneby/RIOT,aeneby/RIOT,x3ro/RIOT,cladmi/RIOT,jasonatran/RIOT,kYc0o/RIOT,mfrey/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,mtausig/RIOT,rfuentess/RIOT,x3ro/RIOT,jasonatran/RIOT,OTAkeys/RIOT,toonst/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,josephnoir/RIOT,basilfx/RIOT,kaspar030/RIOT,smlng/RIOT,ant9000/RIOT,kaspar030/RIOT,kYc0o/RIOT,yogo1212/RIOT,kaspar030/RIOT,x3ro/RIOT,OTAkeys/RIOT,ant9000/RIOT,kYc0o/RIOT,miri64/RIOT,yogo1212/RIOT,OlegHahm/RIOT,miri64/RIOT,OlegHahm/RIOT,toonst/RIOT,authmillenon/RIOT,aeneby/RIOT,rfuentess/RIOT,OlegHahm/RIOT,cladmi/RIOT,rfuentess/RIOT,OTAkeys/RIOT,ant9000/RIOT,cladmi/RIOT,RIOT-OS/RIOT,josephnoir/RIOT,cladmi/RIOT