[platforms/fbdev] Handle error conditions more gracefully

Summary:
So far if the framebuffer platform run into an error on initialization
it did not continue and caused the system to freeze. With this change
it properly emits the initFailed signal in all error conditions which
causes kwin_wayland to terminate. This is a much better situation than
just staying in a running, but frozen state.

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2149