HDF5DataLayer cleanup

This PR is for minor cleanup of HDF5DataLayer:

- less noise during training/testing by changing all the `LOG`s that can happen during `Forward` to `DLOG`s
- die on failure to open an HDF5 file to match the behavior of all other DataLayers -- not sure why it wasn't this way already, was that intentional? (Currently Net::Init will happily continue creating the net even if the HDF5DataLayer produces nothing, and then will probably die once some other layer complains about receiving 0x0x0x0 input..)