Give fatal error dialog an icon.

Not using IconManager to minimise dependencies and reduce causes
of further errors.

Change-Id: I1e8dbfab5705e9965ee24c05f116891271b87326
Fixes-Issue: CLIENT-526
Reviewed-on: http://gerrit.dmdirc.com/3650
Reviewed-by: Chris Smith <chris@dmdirc.com>
Automatic-Compile: DMDirc Build Manager