Revert "Fixed bug caused by old python version."

This reverts commit 4372f54399e50c556b272e0762c073786f16d72d.
Revert "Reformated pylearn2/datasets/exc.py in order to display the environment variable error message in a clean fashion."

This reverts commit cf57e82c380328285bab5c75d80c0f1081f7d31e.
Created file common_strings.py which stores strings common to pylearn2/datasets/exc.py and pylearn2/utils/string_utils.py.