Twirp::Exception to be raised if needed

[marioizquierdo]

Fixes #36

Allow to convert a Twirp::Error into a Twirp::Exception, that is an actual exception extended from StandardError. This enables the possibility of raising exceptions from client errors like this:

begin
  resp = client.do_stuff(foo: "bar")
  raise resp.error.to_exception if resp.error
  # ... do stuff with resp.data

rescue Twirp::Exception => e
  case e.code
  when :not_found
    # render not found page
  when :invalid_argument, :permission_denied, :internal
    # etc ...
  end
end

[marioizquierdo]

Reverted on PR #43, we will not implement exceptions, as per discussed in issue #36