add check to make sure gem build fails when generated jars not present
add check to make sure gem build fails when generated jars not present

Now when you run `gem build mirah.gemspec` and the bootstrap jar has not been compiled, it will complain, tell you to use `rake gem` and quit

mirah/mirah,KeenS/mirah,uujava/mirah,uujava/mirah,mirah/mirah,felixvf/mirah,KeenS/mirah,aspleenic/mirah,mirah/mirah,mirah/mirah,KeenS/mirah,uujava/mirah,aspleenic/mirah,uujava/mirah,aspleenic/mirah,KeenS/mirah,aspleenic/mirah,felixvf/mirah,felixvf/mirah,felixvf/mirah,aspleenic/mirah,uujava/mirah