Fixes https://github.com/spring-projects/spring-integration/issues/2760 * The `assertThatExceptionOfType()` is more convenient, than `assertThatThrownBy`, so, replace all the usages accordingly * Fix JavaDoc typo in the `AbstractScriptExecutingMessageProcessor` * Fix Sonar smells for `throws Exception` in `AbstractScriptExecutingMessageProcessor` hierarchy and some code polishing for them