nicer static_xml::ser::Error

[scottlamb]

I've written a nice static_xml::de::Error type which outputs some context information, including the current XML stack. static_xml::ser::Error should do the same.

Likely needs similar instrumentation in which some things return this type, text-based stuff returns BoxedStdError (as they don't have the proper context to fill this; they need to rely on the caller to do so), and some things return either type (see static_xml::de::VisitorError).