Rework error handling, depend on Twisted Words 0.4.0.
Twisted Words 0.4.0 introduced support for stanza error handling, much better
than the custom error handling in Idavoll. Also, all protocol-level errors
were examined and brought up to date with version 1.8 of JEP-0060.
As a result of the error examination, the retrieval of default configuration
options using <default/> is now supported properly.