Fix issue #85 - Need public interface for finding global module path

Now we implement the interface of no path string when the module is not found. Maybe in the future we will need to return error codes to indicate more things.
For now we just return "".

Testing:
ran mbs-mod fg badname, saw nothing echoed
ran mbs-data-set badname bla/bla/bla, saw error about badname module not exisitng