Add better error feddback to ClasspathUpdaterTest

Currently the ClasspathUpdaterTest just complains that the update has
failed but the user has no chance to see what's wrong.

Thats especially annoying as the test randomly fails.

ui/org.eclipse.pde.ui.tests/src/org/eclipse/pde/ui/tests/classpathupdater/ClasspathUpdaterTest.java

@@ -26,6 +26,7 @@
 
 import org.eclipse.core.resources.IProject;
 import org.eclipse.core.runtime.IPath;
+import org.eclipse.core.runtime.IStatus;
 import org.eclipse.core.runtime.jobs.Job;
 import org.eclipse.jdt.core.IClasspathAttribute;
 import org.eclipse.jdt.core.IClasspathEntry;
@@ -180,7 +181,8 @@ private void runUpdateClasspathJob() throws InterruptedException {
 		IPluginModelBase model = PluginRegistry.findModel(project.getProject());
 		Job job = UpdateClasspathJob.scheduleFor(List.of(model), false);
 		job.join();
-		assertTrue("Update Classpath Job failed", job.getResult().isOK());
+		IStatus result = job.getResult();
+		assertTrue("Update Classpath Job failed: " + result, result.isOK());
 	}
 
 	private <T> void assertClasspathAttribute(String entryName, String attrName, T expectedValue,