Fixup Introduce nested exception printing

Author: thk123

src/util/unwrap_nested_exception.cpp

@@ -42,7 +42,7 @@ std::string unwrap_exception(const std::exception &e, int level)
     std::string nested_message = unwrap_exception(e, level + 1);
     // Some exception messages already end in  a new line (e.g. as they have
     // dumped an irept. Most do not so add a new line on.
-    if(!has_suffix(nested_message, "\n")) // TODO: replace with more C++ style
+    if(nested_message.back() != '\n')
     {
       message_stream << '\n';
     }