Verbose plugin not found exception (#849)

* Add Plugin name for verbose Plugin not found exception
* Make the plugin loading failure exception more verbose
* Throw Opensearch in place of RuntimeException for plugin load failure
* Nit fix, added ... to make logging standout

Signed-off-by: Jayesh Hathila <sharma.jayesh52@gmail.com>

server/src/main/java/org/opensearch/plugins/PluginsService.java

@@ -662,6 +662,7 @@ public class PluginsService implements ReportingService<PluginsAndModules> {
                 return null;
             });
 
+            logger.debug("Loading plugin [" + name + "]...");
             Class<? extends Plugin> pluginClass = loadPluginClass(bundle.plugin.getClassname(), loader);
             if (loader != pluginClass.getClassLoader()) {
                 throw new IllegalStateException("Plugin [" + name + "] must reference a class loader local Plugin class ["
@@ -700,8 +701,8 @@ public class PluginsService implements ReportingService<PluginsAndModules> {
     private Class<? extends Plugin> loadPluginClass(String className, ClassLoader loader) {
         try {
             return Class.forName(className, false, loader).asSubclass(Plugin.class);
-        } catch (ClassNotFoundException e) {
+        } catch (Throwable t) {
-            throw new OpenSearchException("Could not find plugin class [" + className + "]", e);
+            throw new OpenSearchException("Unable to load plugin class [" + className + "]", t);
         }
     }
 

server/src/test/java/org/opensearch/plugins/PluginsServiceTests.java

@@ -734,6 +734,30 @@ public class PluginsServiceTests extends OpenSearchTestCase {
             TestPlugin.class.getName() + "] (class loader [" + PluginsServiceTests.class.getClassLoader() + "])")));
     }
 
+    public void testPluginLoadFailure() throws IOException {
+        final Path pathHome = createTempDir();
+        final Path plugins = pathHome.resolve("plugins");
+        final Path fake = plugins.resolve("fake");
+
+        PluginTestUtil.writePluginProperties(
+            fake,
+            "description", "description",
+            "name", "fake",
+            "version", "1.0.0",
+            "opensearch.version", Version.CURRENT.toString(),
+            "java.version", System.getProperty("java.specification.version"),
+            "classname", "DummyClass"); // This class is not present in Path, hence plugin loading will throw ClassNotFoundException
+
+        final Settings settings =
+            Settings.builder()
+                .put("path.home", pathHome)
+                .put("plugin.mandatory", "fake")
+                .build();
+        RuntimeException exception = expectThrows(RuntimeException.class, () -> newPluginsService(settings));
+        assertTrue(exception.getCause() instanceof ClassNotFoundException);
+        assertThat(exception, hasToString(containsString("Unable to load plugin class [DummyClass]")));
+    }
+
     public void testExtensiblePlugin() {
         TestExtensiblePlugin extensiblePlugin = new TestExtensiblePlugin();
         PluginsService.loadExtensions(Collections.singletonList(