eglconvenience/eglfs: Clean up warning messages

Remove the last occurrence of ifdefed debug prints and replace it with
categorized logging.

Also clean up the warning texts for some of the serious errors, the old
ones from Qt 4 times were somewhat messy.

Change-Id: I2a3e48c393d56be2511d25c3003b1f2b74ac3c8c
Reviewed-by: Louai Al-Khanji <louai.al-khanji@digia.com>
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>