[packagekit] yum backend free filter

[Tim Lauridsen]

> 
> The Tracebacks can be put into error signals, but i have find a good way 
> to add it without making the code crappy ( try / except blocks around 
> all functions )
> 
> 

OK, i have made a custom exception handler there convert a unhandled 
python exception into a packagekit error signal.
I think we need to work a little with the format, so we can present it 
nicely in the frontend.

The first line is
Error Type: <exception class>
second line:
Error Value: <verbose error message>

following lines is the call stack trace:

It is easy to change the format to make it easier to interpret in the 
frontend.

It should work for all python backend there is using 
'PackageKitBaseBackend' as there parent class.

Tim