Make better error message for Q_ENUM with lower-case value names

I created a Q_GADGET class with an enum inside and registered it with QML, but I formatted the enumeration value names as lowerCamelCase instead of UpperCamelCase. Everything worked as anticipated, except that any reference to `GadgetName.enumValue` in QML (which Qt Creator did correctly autocomplete!) would resolve as undefined.

I spent several hours trying to figure out what I'd done wrong before finally stumbling upon a StackOverflow page which mentioned that the value names had to start with an upper-case letter.

Some warning about this (possibly from moc?) would have been nice, as it would have saved me several hours and substantial aggravation. At the very least, this caveat should be mentioned at https://doc.qt.io/qt-5/qobject.html#Q_ENUM