Improved handling of unexpected errors

[idpaterson]

From @aetjansen on #136:

A more meaningful error would be handy, maybe include a reference to Wunderlist's status url?

It might be possible to add a wl-error route and trigger Alfred to pop back up to show it. The error information would need to be stored in a file for retrieval by the handler. With this setup, the handler could show more information about the error, link to the Wunderlist status page, and link to the GitHub issues list to submit a report.