WIP Replace ASSERT by real check

[mgautierfr]

Fixes #670

[kelson42]

@mgautierfr This PR is alsmot 8 months old as it was pretty time consuming to finish it. I had two days of work in my mind. Is that correct?

[mgautierfr]

At least yes.
I have put a lot of check in the low level reader using assert but we want here remove the assert (or keep it in debug, but in release is it as remove it) and add check at higher level.
This means passing through all places where data (coming from the zim file itself) may be wrong and raise a explicit exception "as soon as possible". But those places are not identified, we will probably have a lot of round trip during the review.