HTML support for components throws exception on unknown HTML tag

If you use HTML to create a label or button,
and if you include an invalid HTML tag (such as <bold>) in the HTML,
the program throws a ClassCastException.

It'd be nice if instead, the program printed an error message
and indicated what tag was in error.

Here's a sample trace:
 > > java.lang.ClassCastException: javax.swing.JLabel
> > at javax.swing.text.html.EditableView.setParent(EditableView.java:95)
> > at javax.swing.text.ParagraphView.loadChildren(ParagraphView.java:130)
> > at javax.swing.text.CompositeView.setParent(CompositeView.java:360)
> > at javax.swing.text.html.ParagraphView.setParent(ParagraphView.java:67)
> > at javax.swing.text.CompositeView.replace(CompositeView.java:185)
> > at javax.swing.text.BoxView.replace(BoxView.java:101)
> > at javax.swing.text.CompositeView.loadChildren(CompositeView.java:99)
> > at javax.swing.text.CompositeView.setParent(CompositeView.java:360)
> > at javax.swing.text.CompositeView.replace(CompositeView.java:185)
> > at javax.swing.text.BoxView.replace(BoxView.java:101)
> > at javax.swing.text.CompositeView.loadChildren(CompositeView.java:99)
> > at javax.swing.text.CompositeView.setParent(CompositeView.java:360)
> > at javax.swing.plaf.basic.BasicHTML$Renderer.<init>(BasicHTML.java:186)
> > at javax.swing.plaf.basic.BasicHTML.createHTMLView(BasicHTML.java:48)
> > at javax.swing.plaf.basic.BasicHTML.updateRenderer(BasicHTML.java:77)
> > at javax.swing.plaf.basic.BasicLabelUI.propertyChange(BasicLabelUI.java:355)
> > at
> > javax.swing.event.SwingPropertyChangeSupport.firePropertyChange(SwingPropertyChangeSupport.java:154)
> > at javax.swing.JComponent.firePropertyChange(JComponent.java:2990)
> > at javax.swing.JLabel.setText(JLabel.java:287)
> >