HSEARCH-3170 Use meaningful messages when nesting exceptions, relying…

… on the cause chain if necessary

InvocationTargetException, in particular, will not show any message,
which used to result in something like "Got exception: null" in the
failure report. Now we will try to get the message for the cause of that
InvocationTargetException, which may be more informative.

engine/src/main/java/org/hibernate/search/engine/environment/classpath/spi/ClassLoaderHelper.java

@@ -16,6 +16,7 @@
 import org.hibernate.search.util.SearchException;
 import org.hibernate.search.util.impl.common.LoggerFactory;
 import org.hibernate.search.util.impl.common.StringHelper;
+import org.hibernate.search.util.impl.common.Throwables;
 
 /**
  * Utility class to load instances of other classes by using a fully qualified name,
@@ -100,10 +101,12 @@ public static <T> T untypedInstanceFromClass(final Class<T> classToLoad, final S
 		}
 		catch (IllegalAccessException | InvocationTargetException | InstantiationException e) {
 			if ( StringHelper.isEmpty( componentDescription ) ) {
-				throw log.unableToInstantiateClass( classToLoad, e.getMessage(), e );
+				throw log.unableToInstantiateClass( classToLoad, Throwables.getFirstNonNullMessage( e ), e );
 			}
 			else {
-				throw log.unableToInstantiateComponent( componentDescription, classToLoad, e.getMessage(), e );
+				throw log.unableToInstantiateComponent(
+						componentDescription, classToLoad, Throwables.getFirstNonNullMessage( e ), e
+				);
 			}
 		}
 	}
@@ -161,7 +164,9 @@ public static <T> T instanceFromClass(Class<T> targetSuperType, Class<?> classTo
 			instance = singleMapConstructor.newInstance( constructorParameter );
 		}
 		catch (Exception e) {
-			throw log.unableToInstantiateComponent( componentDescription, classToLoad, e.getMessage(), e );
+			throw log.unableToInstantiateComponent(
+					componentDescription, classToLoad, Throwables.getFirstNonNullMessage( e ), e
+			);
 		}
 		return verifySuperTypeCompatibility( targetSuperType, instance, classToLoad, componentDescription );
 	}

util/internal/common/src/main/java/org/hibernate/search/util/impl/common/Throwables.java

@@ -63,4 +63,13 @@ public static <T extends Throwable> T combine(T throwable, T otherThrowable) {
 		return toThrow;
 	}
 
+	public static String getFirstNonNullMessage(Throwable t) {
+		Throwable cause = t.getCause();
+		while ( t.getMessage() == null && cause != null ) {
+			t = cause;
+			cause = t.getCause();
+		}
+		return t.getMessage();
+	}
+
 }