Consistent logging in annotated exception handler

This commit aligns DEBUG logging of exceptions resolved via annotated
exception handler methods with ExceptionResolversExceptionHandler and
the reference documentation.

Closes gh-996

Author: rstoyanchev

spring-graphql/src/main/java/org/springframework/graphql/data/method/annotation/support/AnnotatedControllerExceptionResolver.java

@@ -238,7 +238,17 @@ private Mono<List<GraphQLError>> invokeExceptionHandler(
 
 			Object result = exceptionHandler.invoke(env, arguments);
 
-			return methodReturnValueAdapter.adapt(result, exception);
+			Mono<List<GraphQLError>> errorsMono = methodReturnValueAdapter.adapt(result, exception);
+			if (logger.isDebugEnabled()) {
+				errorsMono = errorsMono.doOnNext((errors) -> {
+					if (logger.isDebugEnabled()) {
+						String name = exception.getClass().getSimpleName();
+						logger.debug("Resolved " + name + " to GraphQL error(s): " + errors, exception);
+					}
+				});
+			}
+
+			return errorsMono;
 		}
 		catch (Throwable invocationEx) {
 			// Any other than the original exception (or a cause) is unintended here,