Make error raising consistent for iterative affine co-registration in case of divergence

[rhugonnet]

For most AffineCoreg classes, sampled valid values change at each iteration (NaNs propagate depending on iterative shift/rotation).
Currently, only NuthKaab has an error raised in that case:

xdem/xdem/coreg/affine.py

Line 464 in 22dbe7c

raise ValueError(

We should make this sampling error due to divergence consistent across all these functions (ICP, DhMinimize, etc), maybe as a common error class DivergenceError. Error catching will be different for methods who are iterative within their minimizer (like DhMinimize) and not within xDEM, there we'll need to catch an exception.

And we should also add tests for this: Not easy to reproduce, as the methods can find local minima without always completely diverging to the point of having no more samples...