DefaultUIService: give a hint why no UIs are found

Multiple people have now been confused by this error.
So let's give a suggestion how to overcome it.

See:
* http://forum.imagej.net/t/basic-imagej2-question-how-to-open-imagej/1179
* http://forum.imagej.net/t/register-virtual-stack-slices-plugin-hangs-on-macos/1123/12

Author: ctrueden

src/main/java/org/scijava/ui/DefaultUIService.java

@@ -159,9 +159,7 @@ public void addUI(final String name, final UserInterface ui) {
 	public void showUI() {
 		if (disposed) return;
 		final UserInterface ui = getDefaultUI();
-		if (ui == null) {
-			throw new IllegalStateException("No UIs available.");
-		}
+		if (ui == null) throw noUIsAvailableException();
 		showUI(ui);
 	}
 
@@ -188,9 +186,7 @@ public void showUI(final UserInterface ui) {
 	@Override
 	public boolean isVisible() {
 		final UserInterface ui = getDefaultUI();
-		if (ui == null) {
-			throw new IllegalStateException("No UIs available.");
-		}
+		if (ui == null) throw noUIsAvailableException();
 		return ui.isVisible();
 	}
 
@@ -547,4 +543,10 @@ private void addUserInterface(final String name, final UserInterface ui) {
 	private String getTitle() {
 		return appService.getApp().getTitle();
 	}
+
+	private IllegalStateException noUIsAvailableException() {
+		return new IllegalStateException("No UIs available. " +
+			"Please add a component containing a UIPlugin " +
+			"(e.g., scijava-ui-swing) to your class-path.");
+	}
 }