Another set of improvements to the error code

This time the changes are all localized in arti_client. They are:

• Make detail() an accessor function.
• Rename Error to ErrorDetail.
• Rename TorResult to Result.
• Simplify visibility macros a bit.
• Update launch of chan expiration task to new API

The final commit fixes a compilation error on main; unfortunately it doesn't backport cleanly without the rest of the branch. Let me know if I should do an interim fix.

Closes #321 (closed).

changed milestone to %Arti 0.1.0 release: Okay for experimental embedding

requested review from @eta

assigned to @nickm

approved this merge request

merged

mentioned in commit ba2eb765