client: throw non-structured response errors as internal errors

Some servers return errors in a non-v1.1+ compliant manner, as text.
An example in the wild is electrs (https://github.com/romanz/electrs).

This behaviour was apparently common in pre-v1.1 servers, and since it
is easy to support, throw the error with its text so the caller at least
gets some error context rather than "invalid server response".

Author: jgriffiths

include/jsonrpccxx/client.hpp

@@ -59,6 +59,8 @@ namespace jsonrpccxx {
         json response = json::parse(connector.Send(j.dump()));
         if (has_key_type(response, "error", json::value_t::object)) {
           throw JsonRpcException::fromJson(response["error"]);
+        } else if (has_key_type(response, "error", json::value_t::string)) {
+          throw JsonRpcException(-32603, response["error"]);
         }
         if (has_key(response, "result") && has_key(response, "id")) {
           if (response["id"].type() == json::value_t::string)