Handle application script errors consistently.

[samuel-williams-shopify]

Summary

• define a shared application exception boundary for exception handling middleware
• handle and report ScriptError failures such as SyntaxError consistently
• allow process-level exceptions such as Interrupt to propagate
• cover both handler and mailer behavior

Closes #25.

Testing

• bundle exec sus test/utopia/exceptions/handler.rb
• bundle exec sus test/utopia/exceptions/mailer.rb
• bundle exec rubocop lib/utopia/exceptions/application_errors.rb lib/utopia/exceptions/handler.rb lib/utopia/exceptions/mailer.rb test/utopia/exceptions/.handler/controller.rb test/utopia/exceptions/handler.rb test/utopia/exceptions/mailer.rb
• bundle exec bake decode:index:coverage lib