Add assertion to fail more clearly

if the file is missing for whatever reason

src/test/java/io/jenkins/plugins/evergreen/EvergreenUpdateLevelPageDecoratorTest.java

@@ -3,14 +3,17 @@
 import org.junit.Test;
 
 import java.io.File;
+import java.net.URL;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
 public class EvergreenUpdateLevelPageDecoratorTest {
     @Test
     public void getUpdateLevel() {
 
-        final File manifestFile = new File(getClass().getResource("manifest.json").getPath());
+        final URL resource = getClass().getResource("manifest.json");
+        assertThat(resource).isNotNull();
+        final File manifestFile = new File(resource.getPath());
         assertThat(manifestFile).exists();
         assertThat(new EvergreenUpdateLevelPageDecorator(manifestFile).getUpdateLevel()).isEqualTo("2");
     }