If there are pending migrations, tell the app to shut down

[jwoertink]

Fixes #671
Fixes luckyframework/lucky#1142

This PR adds a front and center error when there's pending migrations.

Before PR:

After PR:

there is one minor issue still here which is if you boot your app locally without a process manager (e.g. crystal src/start_server.cr), it will try to send interrupt signals to the parent process.

Most likely this isn't a big issue. You can even just run lucky watch and it's fine. It's only when booting the app manually through crystal.

[robacarp]

Does this set of changes alter how a booting production-mode app would behave when database migrations are pending?

[jwoertink]

Well, by default, this would only run in development. For production, you have a few different options of either building the migrate task and manually running that, or changing the linked code to look more like:

if LuckyEnv.development?
  Avram::Migrator::Runner.new.ensure_migrated!
  Avram::SchemaEnforcer.ensure_correct_column_mappings!
else
  Avram::Migrator::Runner.new.run_pending_migrations
end

but the ensure_migrated! method would still only run for development.