sstate.bbclass: Do not cause build failures due to setscene errors

sstate.bbclass: Do not cause build failures due to setscene errors

If a setscene task fails, the real task will be run instead. However,
in case the failed setscene task happened to log any errors, this will
still cause bitbake to return with an error code, even though
everything actually built ok. To avoid this, modify setscene to only
warn about errors.

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>