Made the error condition when Yakuake is unable to load the Konsole KPart component non-destructive, i.e. being unable to load the KPart will no longer end in an unavoidable application quit.

Instead, a good-looking warning will be shown where the terminal would normally be.

This also takes care of the edge case in which Konsole is uninstalled while Yakuake is already running, so that existing terminals won't be lost.