Bug 299208 [CommonNavigator] More helpful error message when an invalid suppressedExtensionId is referenced
The current error message does not show the if of the NCE that had the bad reference.
bundles/org.eclipse.ui.navigator/src/org/eclipse/ui/internal/navigator/extensions/NavigatorContentDescriptorManager.java
if (desc.hasSaveablesProvider()) {
					saveablesProviderDescriptors.add(desc);
					if (Policy.DEBUG_EXTENSION_SETUP) {
						System.out.println("Saveables provider descriptor: " + desc); 
					}
				}
			}
		}
	}
	/**
	 * 
	 */
	private void computeOverrides() {
		if (overridingDescriptors.size() > 0) {
			NavigatorContentDescriptor descriptor;
			NavigatorContentDescriptor overriddenDescriptor;
			for (Iterator overridingIterator = overridingDescriptors.iterator(); overridingIterator
					.hasNext();) {
				descriptor = (NavigatorContentDescriptor) overridingIterator
						.next();
				overriddenDescriptor = (NavigatorContentDescriptor) allDescriptors
						.get(descriptor.getSuppressedExtensionId());
				if (overriddenDescriptor != null) {
					if (Policy.DEBUG_EXTENSION_SETUP) {
						System.out.println(descriptor + " overrides: " + overriddenDescriptor); 
					}
					/*
					 * add the descriptor as an overriding extension for its
					 * suppressed extension
					 */