serve(urlString:) should accept errorHandler closure

[Toshihiro Suzuki]

If serve(urlString:type:) or serve(urlString:... for multi window... ) fails, currently there is no way to recover.
User will be left with current screen (or empty Kitchen.window for multi window case).

The new error handler should be called on

• XML parse error
• Network error

I don't think of other errors than above, but if there are other kind of errors, they will be ignored.
Note that all errors are reported to Cookbook.onError.