Qbs: Fallback to filename as project name if qbs parsing fails

Use the project filename as a fallback in case qbs parsing fails
without being able to extract the main project data.

Task-number: QTCREATORBUG-9777
Change-Id: Iceedf55c7f5d56a8876c19bcbd94082752c08336
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>