wayland-egl: Logging cleanup.

Prefix warnings with QtCompositor so it's clear where they are coming from.
Remove trailing newlines from messages, they aren't needed.
Also, only warn if something actually goes wrong.

Change-Id: I88b8b03480d938a08a84a4964108d10d86606be6
Reviewed-by: Giulio Camuffo <giulio.camuffo@jollamobile.com>