Add `return` option to `#error_handling`

[ryz310]

Some API returns 404 Not Found as normal operation.
In the case, the error handler raises error is not kindness.

I hope to return nil or false in that case.

# Example.
class SomeApiClient < MyApiClient::Base
  endpoint 'https://example.com'

  error_handling status_code: 404, return: nil

  def get_resource
    get 'path/to/resource'
  end
end

api_client = SomeApiClient.new
response = api_client.get_resource # 404 Not Found
# => nil