Enable `errcheck` linter

[peterstace]

Description

Enables the errcheck linter, which checks that all errors are handled.

Check List

Have you:

• Added unit tests? N/A

• Add cmprefimpl tests? (if appropriate?) N/A

• Updated release notes? (if appropriate?) N/A

Related Issue

• https://github.com/peterstace/simplefeatures/issues/522

[peterstace]

Thanks for the review @albertteoh 🙇