CLIMATE-487 - Improve error message when local dataset load fails

- Explicit RuntimeErrors are not properly handled to alert the user that
  the supplied file path is invalid. This helps clarify user
  controllable failures.
- Attempt to make the generic error on dataset loading a bit more
  descriptive to help users.
CLIMATE-371 - Allow user to pass variable names to local.py
Resolve CLIMATE-487. Merge PR #131.
Resolve CLIMATE-371. Merge PR #132.