AppStream::Pool::load() returns false on some systems

[aleixpol]

At least OpenSuse, Neon and Kubuntu (with PPAs).

Reference: https://bugs.kde.org/show_bug.cgi?id=374279

[ximion]

That is expected behavior, it will return TRUE only if all data was loaded, if only one bit of data was ignored, FALSE is returned.
To check if there was a bigger error, the function will set a GError - we should expose the error message too on the Qt bindings (add a lastError QString property? or do something more fancy?).

Changing the meaning of the load() return value could break things, doing so would only make sense with some prior announcement (it would at least break appstreamcli, which uses it to show a warning message (but no hard failure) if data wasn't loaded. The function returning false is usually a hint for more experienced users to search for issues, non-technically advanced users should ignore it, since the issues is almost always in upstream software or an issue with the distributor)