Make handle_error work on sub-applications

[TangentFoxy]

trafficstars

I recall some features of the main application do not work on sub-applications, is this one of them?

[leafo]

handle_error must be defined on the parent application. The error handler of the sub application is ignored when it's merged into a parent one.

[TangentFoxy]

In that case, this is now a feature request, haha.

For now though, I will rewrite things to work around that.