scilib/lsqmdl.py: rename 'nterms' yet again.

I can never remember whether `nterms = 1` means y = ax + b or just y = b. New
name is `maxexponent`, which is offset from `nterms` by 1, but I feel is
definitively clear.
quickutils/utils/reraise_context.py: new

The first new one in quite a while. I think I'm going to like this one.