Display a normal link if we can't display an image.

When the value of an annotation is an IRI that seems to point to an
image, and we are set to display thumbnails, check if we were able to
sucessfully load the remote image; if not, then fallback to display the
link, as if the "Display thumbnails for image URLs" option was disabled.

protege-editor-owl/src/main/java/org/protege/editor/owl/ui/renderer/OWLAnnotationCellRenderer2.java

@@ -419,7 +419,11 @@ private List<Paragraph> renderExternalIRI(Page page, IRI iri) {
             try {
                 if (isImageAddress(iri) && isDisplayThumbnails()) {
                     IconBox iconBox = getImageBox(iri);
-                    page.add(iconBox);
+                    if (iconBox != null) {
+                        page.add(iconBox);
+                    } else {
+                        paragraphs.add(page.addParagraph(iriString, new HTTPLink(iri.toURI())));
+                    }
                 }
                 else {
                     paragraphs.add(page.addParagraph(iriString, new HTTPLink(iri.toURI())));
@@ -469,7 +473,9 @@ public void setThumbnailRendering(InlineThumbnailRendering thumbnailRendering) {
      */
     private IconBox getImageBox(IRI iri) throws MalformedURLException, IllegalArgumentException {
         ImageIcon imageIcon = new ImageIcon(iri.toURI().toURL());
-        imageIcon.getImageLoadStatus();
+        if ( imageIcon.getImageLoadStatus() != MediaTracker.COMPLETE ) {
+            return null;
+        }
         IconBox iconBox = new IconBox(imageIcon, new HTTPLink(iri.toURI()));
         iconBox.setMaxHeight(50);
         return iconBox;