Replace eprintln! with logging

[ratijas]

There is a call to eprintln! macro within singleton registration callback. It seems like a good place for logging frameworks instead. We could use error! from the Rust logging facade or qFatal from Qt + cpp! side. Since rust logging facade is feature-flagged, probably calling Qt would make more sense, or feature-flag double-implementation to avoid round-trips when rust logging is enabled.

qmetaobject-rs/qmetaobject/src/qtdeclarative.rs

Lines 489 to 492 in 582ce49

	Err(_panic) => {
	eprintln!("qml_register_singleton_type T::default or T::init panicked.");
	std::process::abort()
	}

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By the way,

• qCritical (citing) "exits if the environment variable QT_FATAL_CRITICALS is not empty", and
• qFatal (citing) "using the default message handler this function will abort to create a core dump".

I don't remember implementing aborts in rust message handler in #86 🤔