Wrap validation errors

[htdvisser]

Summary:

This PR wraps the validation errors introduced in #194 so that they can be communicated over gRPC.

Changes:

As an additional change, I fixed the formatting of error cause stacks in the CLI

Notes for Reviewers:

This is technically an API change, because we now start communicating these errors over GRPC if they are the cause of an RPC to fail. However, these causes are only informative, so I didn't attach the compat/api label.

% go run ./cmd/ttn-lw-cli app create admin_app --user-id admin
error:pkg/ttnpb:identifiers (invalid identifiers)
    correlation_id=50b56c3f59654d68b5f52edb37302f32
--- error:pkg/errors:validation (invalid `application_id`: value does not match regex pattern "^[a-z0-9](?:[-]?[a-z0-9]){2,}$")
    name=ApplicationIdentifiersValidationError
    field=application_id
    reason=value does not match regex pattern "^[a-z0-9](?:[-]?[a-z0-9]){2,}$"
exit status 255