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