xwayland: Replace criticalError() signal with a less fatal signal

If the Xwayland executable can't be found, the whole session will die
because a criticalError() signal will be emitted.

This change replaces the criticalError() signal with a less severe
signal.

If the errorOccurred() signal has been emitted during the startup
sequence, kwin won't die and will just continue with spawning the
session process.