Adding warnings to experiments.py

- Added warnings if `hasher_features` is set but `feature_hasher` is either missing or set to False.
Modified experiments.py

- Added specific check for an empty featureset list
- Tweaked ValueError strings
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Added more config parsing tests

- Added tests to check errors for parsing sampler, feature hashing, featuresets and featureset names.
Modified empty featureset check

- Turns out that the featureset list can be empty if `train_file` and `test_file` are specified. Updated accordingly.