QDoc: No warning if \qmltype contains \inherits with non-existing type

When documenting a QML type with the \qmltype command, one can describe an inheritance relationship to another type with the \inherits command. If the argument to the latter command is a type that doesn't exist, QDoc doesn't issue any sort of warning or debug message. This can make it tricky to track down why what an author may expect as output doesn't actually appear.