Remove Deprecation Warning associated with .load command when there are no files at all

[rstoneback]

See pull #1179. An IndexError results when trying to load data when there are no files at all. There is a DeprecationWarning that in pysat version 3.3 the IndexError will not be raised. Remove this warning. Further, add a return to command to exit the function early so the error is no longer raised. Retain the helpful warning geared towards new users that pysat didn't find any files at all.