Add the ability to rescue from specific exception in handlers

[ellmetha]

Description

Let's add the ability to specify exceptions to rescue from in handlers by defining a new macro similar to Rails' rescue_from.

Rationale

Presently, the framework does not allow to easily declare exceptions to rescue from in handlers. The only way to do that is to implement some custom rescue logic in the #dispatch or HTTP method-specific method (such as #get or #post for example).

Proposition

Let's add a new #rescue_from macro to handlers that will allow defining specific exceptions that need to be rescued as well as the name of a method that must be executed when these exceptions are indeed rescued:

class ProfileHandler < Marten::Handlers::Template
  include RequireSignedInUser

  rescue_from Auth::UnauthorizedUser, with: :handle_unauthorized_user

  template_name "auth/profile.html"

  private def handle_unauthorized_user
    head :forbidden
  end
end

If the specific callback returns a valid HTTP response, then it should be used as is and returned by the handler. Obviously the callback could also raise a different kind of error, but if the handler returns a value that is not an HTTP response then the original exceptions should be re-raised.