```ruby
require "active_support/notifications"
require "stripe"
require "stripe_event/engine" if defined?(Rails)

module StripeEvent
  class << self
    attr_accessor :adapter, :backend, :event_retriever, :namespace

    def setup(&block)
      instance_eval(&block)
    end

    def instrument(params)
      begin
        event = event_retriever.call(params)
      rescue Stripe::StripeError => e
        raise UnauthorizedError.new(e)
      end

      backend.instrument namespace.call(event[:type]), event
    end

    def subscribe(name, callable = Proc.new)
      backend.subscribe namespace.to_regexp(name), adapter.call(callable)
    end

    def all(callable = Proc.new)
      subscribe nil, callable
    end
  end

  class Namespace < Struct.new(:value, :delimiter)
    def call(name = nil)
      name ? "#{value}#{delimiter}#{name}" : value
    end

    def to_regexp(name = nil)
      %r{^#{Regexp.escape call(name)}}
    end
  end

  class NotificationAdapter < Struct.new(:subscriber)
    def self.call(callable)
      new(callable)
    end

    def call(*args)
      payload = args.last
      subscriber.call(payload)
    end
  end

  class StripeEvent::Error < StandardError; end
  class StripeEvent::UnauthorizedError < StripeEvent::Error; end

  self.adapter = NotificationAdapter
  self.backend = ActiveSupport::Notifications
  self.event_retriever = lambda { |params| Stripe::Event.retrieve(params[:id]) }
  self.namespace = Namespace.new("stripe_event", ".")
end
```

Tidy up error class declaration
user
```ruby
require "active_support/notifications"
require "stripe"
require "stripe_event/engine" if defined?(Rails)

module StripeEvent
  class << self
    attr_accessor :adapter, :backend, :event_retriever, :namespace

    def setup(&block)
      instance_eval(&block)
    end

    def instrument(params)
      begin
        event = event_retriever.call(params)
      rescue Stripe::StripeError => e
        raise UnauthorizedError.new(e)
      end

      backend.instrument namespace.call(event[:type]), event
    end

    def subscribe(name, callable = Proc.new)
      backend.subscribe namespace.to_regexp(name), adapter.call(callable)
    end

    def all(callable = Proc.new)
      subscribe nil, callable
    end
  end

  class Namespace < Struct.new(:value, :delimiter)
    def call(name = nil)
      name ? "#{value}#{delimiter}#{name}" : value
    end

    def to_regexp(name = nil)
      %r{^#{Regexp.escape call(name)}}
    end
  end

  class NotificationAdapter < Struct.new(:subscriber)
    def self.call(callable)
      new(callable)
    end

    def call(*args)
      payload = args.last
      subscriber.call(payload)
    end
  end

  class Error < StandardError; end
  class UnauthorizedError < Error; end

  self.adapter = NotificationAdapter
  self.backend = ActiveSupport::Notifications
  self.event_retriever = lambda { |params| Stripe::Event.retrieve(params[:id]) }
  self.namespace = Namespace.new("stripe_event", ".")
end
```
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