Improve QA pipeline error handling
Author: Narsil
Pull request: yes

# What does this PR do?

- The issue is that with previous code we would have the following:

```python
qa_pipeline = (...)
qa_pipeline(question="Where was he born ?", context="")
-> IndexError: Dimension out of range (expected to be in range of [-1, 0], but got 1)
```

The goal here is to improve this to actually return a ValueError
wherever possible.

While at it, I tried to simplify QuestionArgumentHandler's code to
make it smaller and more compat while keeping backward compat.

Quick note: For the tests, I feel they would be more readable if it was possible to write

```python
self.assertEqual(qa(.....), [SquadExample(None, Q, C, None, None,...)])
```
as it would cover both types, and length and deep equality.
However, it's not possible because SquadExample does not implement `__eq__`. It felt out of scope, but 
if reviewers think it would be a nice addition, I'd be happy to implement it and change the test.



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## Who can review?

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@mfuntowicz
@LysandreJik

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