Checker error template overrides

[perryqh]

Why is this change being made?

Allowing for checker violation error message overrides so that consumers can provide better contextual error messages.

What is this change doing?

Allows for new packwerk.yml checker_overrides.

Example

packwerk.yml

checker_overrides:
  privacy_error_template: "{{reference_location}}Privacy violation: `{{constant_name}}` is private to `{{defining_pack_name}}`, but referenced from `{{referencing_pack_name}}`. See https://go/pks-privacy"
  dependency_error_template: "{{reference_location}}Dependency violation: `{{constant_name}}` belongs to `{{defining_pack_name}}`, but `{{referencing_pack_relative_yml}}` does not specify a dependency on `{{defining_pack_name}}`. See https://go/pks-dependency"