check: Switch from simple error types with go-multierror to diagnostic-based error handling

[bflad]

Currently, the check command is very terminal-based and checks use wrapped errors with https://github.com/hashicorp/go-multierror to gather multiple errors for returning the results. To provide better extensibility in the future, e.g. JSON based error results, the underlying checks should implement their own diagnostic types with severity and message fields.

[bflad]

This project will be archived in preference of the revamped validate command in github.com/hashicorp/terraform-plugin-docs implementing much of the basic validation logic from this project. Feature requests should be submitted over there at this point.