Error reporting in the top-level Makefile loop

I've added a line to check the success/failure of each internal make 'subcommand' from the top-level Makefile.

From a scripted build standpoint, it's otherwise difficult to tell if the build succeeded or not, since the top-level Makefile swallows the value of each internal make call:
    
    $(MAKE) --no-print-directory -C $$dir; \

and just always returns a zero return value, whether any individual step failed or not. 

I'm adding this for gkno, feel free to use or ignore for Mosaik in general.