#12789 Replace assert statements with exceptions
# What does this PR do?

Replaces the assertions in utils_qa.py file at examples/tensorflow/question-answering, examples/flax/question-answering and examples/pytorch/question-answering (bound by strict copy check) with ValueError and EnvironmentError exceptions as necessary. 
Also fixed a typo changing predicitions -> predictions in postprocess_qa_predictions_with_beam_search function.

Contributes towards fixing issue [#12789 ](https://github.com/huggingface/transformers/issues/12789)

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?

Can someone please take a look at my PR. The error log mentions a config.json file to be missing in a tmp directory and then goes on to throw 2 assertion errors. But the files where I have made my changes are not mentioned in the logs anywhere. I'm having trouble understanding why my changes may have caused a missing config file. Sorry about the force-push, it was required for a little cleanup.<|||||>Thanks again!