QtIFW

Fix annoying warning printed during installation.

Make the Error class to not inherit from runtime error,
there's no reason to do so. Add empty ctor to avoid dumping
the error message at least once. Check if a .dat file exists
and only then print warning.

Change-Id: I97833947022e05ee3b14992c8ec662874fb5d343
Reviewed-by: Christoph Vogtländer <c.vogtlaender@sigma-surface-science.com>
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>
Reviewed-by: Niels Weber <niels.weber@digia.com>