Add `geojson::Result<T>` and apply it to codebase

[michaelkirk]

• [x] I agree to follow the project's code of conduct.

• [x] I added an entry to CHANGES.md if knowledge of this change could be valuable to users.

This is a pretty superficial change, but I think it's nice.

[michaelkirk]

bors r=frewsxcv,lnicola

[bors[bot]]

Build succeeded:

• build_and_test