```ruby
require 'attrs/version'

def Attrs(*args, &block)
  Attrs.new(*args, &block)
end

module Attrs
  def self.new(*attribute_names, &block)
    Class.new do
      const_set(:ATTRIBUTE_NAMES, attribute_names)

      def self.attribute_names
        self::ATTRIBUTE_NAMES
      end

      def self.attr(name)
        attr_accessor name
        private :"#{name}="
      end

      attribute_names.each { |name| attr name }

      def initialize(attributes)
        self.class.attribute_names.each do |name|
          send("#{name}=", attributes.fetch(name) {
            if respond_to?("default_#{name}", true)
              send "default_#{name}"
            else
              raise KeyError, "key not found: :age. Define :default_#{name} for a default value"
            end
          })
        end
      end

      def inspect
        "#<#{self.class.name} #{attributes.inspect}>"
      end

      def ==(other)
        to_hash == other.to_hash
      end

      def attributes
        self.class.attribute_names.each_with_object({}) do |name, hash|
          hash[name] = send(name)
        end
      end

      alias_method :to_hash, :attributes

      class_eval(&block) if block_given?
    end
  end
end
```

Fix error message raised when attribute is not found
user
```ruby
require 'attrs/version'

def Attrs(*args, &block)
  Attrs.new(*args, &block)
end

module Attrs
  def self.new(*attribute_names, &block)
    Class.new do
      const_set(:ATTRIBUTE_NAMES, attribute_names)

      def self.attribute_names
        self::ATTRIBUTE_NAMES
      end

      def self.attr(name)
        attr_accessor name
        private :"#{name}="
      end

      attribute_names.each { |name| attr name }

      def initialize(attributes)
        self.class.attribute_names.each do |name|
          send("#{name}=", attributes.fetch(name) {
            if respond_to?("default_#{name}", true)
              send "default_#{name}"
            else
              raise KeyError, "key not found: #{name.inspect}. Define :default_#{name} for a default value"
            end
          })
        end
      end

      def inspect
        "#<#{self.class.name} #{attributes.inspect}>"
      end

      def ==(other)
        to_hash == other.to_hash
      end

      def attributes
        self.class.attribute_names.each_with_object({}) do |name, hash|
          hash[name] = send(name)
        end
      end

      alias_method :to_hash, :attributes

      class_eval(&block) if block_given?
    end
  end
end
```
assistant