*Exception.Show used to work differently when the traceback contains one frame
vs more frames. Harmonize how they work, and consistent with how parse errors
and compilation errors are shown.
- Change Context to export all its fields.
- Include end position in Context, and include it in Show.
- Remove the Type field from Error, and express it using an ErrorTag type
parameter instead.
- Make {Pack Unpack}CognateErrors type-safe with the new ErrorTag mechanism, and
rename them to just {Pack Unpack}Errors.
The Error methods used to show the start and end indices, while the Show methods
used to show line ranges.
Showing line:col of the start position seems to be pretty standard; both Go and
Rust do that.
I tried including the line:col of the end position too, but can't find a good
enough format.
When attempting to update a read-only var return an error struct rather
than a simple string (i.e., a Go `error` type). This makes it possible
to include the var name in the error message. This builds on commit
a33ecb2d that highlights the offending var name in the stack trace but
does not include the var name in the error message. With this change the
error message includes the offending var name.
Related #255
