feat: export there are no migrations error

[greeflas]

Hi 👋
I have implemented a CLI command to run migrations. In new projects, I might not have migrations from the start, and in such cases, my command fails.
I want to address this issue by exporting the validation error to be able to check it:

_, err := migrator.Migrate(ctx)

if errors.Is(err, migrator.ErrThereAreNoMigrations) {
	return nil
}

[vmihailenco]

Sorry, but this looks like a minor internal detail that will likely change in future. I don't think this should not be part of the API.