Generic error handling for Tectonic.

This crate provides a generic boxed error type, plus supporting utilities. In particular:

• The basic Error type is an anyhow 1.x boxed Error
• The atry! macro allows simple structured annotations to be added to ? operations
• The a_ok_or! macro allows for annotations to Option::ok_or_else calls.

The tectonic_errors create

This crate is part of the Tectonic project. It provides a basic boxed error type and utilities.

• API documentation.
• Main Git repository.

This implementation forked from Cranko.