proc-macro-error is unmaintained

proc-macro-error's maintainer seems to be unreachable, with no commits for 2 years, no releases pushed for 4 years, and no activity on the GitLab repo or response to email.

proc-macro-error also depends on `syn 1.x`, which may be bringing duplicate dependencies into dependant build trees.

## Possible Alternative(s)

- [manyhow](https://crates.io/crates/manyhow)
- [proc-macro2-diagnostics](https://github.com/SergioBenitez/proc-macro2-diagnostics)