Better error messages

[philtweir]

Implements several closely related features:

• error messages show location of call, not lazy-evaluation, if possible
• custom type errors are more specific
• examples in the custom error messages

[philtweir]

Couple of caveats:

• the non-local traceback implementation was hard, so brings a risk of brittleness - we should keep an eye for any related issues
• only the custom type errors have been improved (other than the non-local context), not the native ones - improving Python's standard way of reporting a missing argument seems out-of-scope for this package, and not "least surprise", but the custom errors have generally been given more detail
• as above, the standard errors have been left alone, but the couple of custom errors where this might help have been given inline examples

[elleryames]

I wonder about this warning. As far as I know dask.distributed is not a dependency of ours here. I can see warning/pointing out failure points for future developers (including our future selves) thinking of extending this package for parallel execution.

Separately, the internal lazy construct is supposed to be library agnostic (so in principle users can use dask, ray, etc backends), so should we perhaps make the warning library agnostic as well?

[elleryames]

Thanks for your patience @philtweir !