Fixed bug which occured when running the tests in datasets/tests/test_hdf5.py.
Made error message more descriptive. Added a few comments.
Fixed Pep8
Small fix to string.
Moved comment to location where it was more generally applicable.
Merge pull request #930 from AlassaneNdiaye/fix_test_hdf5_bug

Fixed bug which occured when running the tests in datasets/tests/test_hdf5.py