hdf5_data_layer.cpp: fix indentation
Cleanup HDF5DataLayer log messages.

-Only use "DLOG" during Forward/Backward for less noise during
training/testing
-Cleanup spacing, capitalization, etc.
Die on failure to load HDF5 data file.
HDF5DataLayer: die on failure to open source file
hdf5_load_nd_dataset_helper: check that dataset exists first
Merge pull request #1191 from jeffdonahue/hdf5data-cleanup

HDF5DataLayer cleanup