Raise exceptions instead of using assertions for control flow
Adding the remaining tasks for this issue to help new code contributors.  
$ cd src/datasets && ack assert -lc 
- [x] commands/convert.py:1
- [x] arrow_reader.py:3
- [x] load.py:7
- [x] utils/py_utils.py:2
- [x] features/features.py:9
- [x] arrow_writer.py:7
- [x] search.py:6
- [x] table.py:1
- [x] metric.py:3
- [x] tasks/image_classification.py:1
- [x] arrow_dataset.py:17
- [x] fingerprint.py:6
- [x] io/json.py:1
- [x] io/csv.py:1 Hi all,
I am interested in taking up `fingerprint.py`, `search.py`, `arrow_writer.py` and `metric.py`. Will raise a PR soon! Let me look into `arrow_dataset.py`, `table.py`, `data_files.py` & `features.py` 
Motivated by https://github.com/huggingface/transformers/issues/12789 in Transformers, one welcoming change would be replacing assertions with proper exceptions. The only type of assertions we should keep are those used as sanity checks.

Currently, there is a total of 87 files with the `assert` statements (located under `datasets` and `src/datasets`), so when working on this, to manage the PR size, only modify 4-5 files at most before submitting a PR.