Show error from cpp-httplib on null response to web service

httplib.h has a helper function to convert Error enumeration to a string

This might help if SSL issues crop up again, or for indicating other
connection errors to user.

src/web_service/web_backend.cpp

@@ -111,7 +111,8 @@ struct Client::Impl {
         httplib::Error error;
 
         if (!cli->send(request, response, error)) {
-            LOG_ERROR(WebService, "{} to {} returned null", method, host + path);
+            LOG_ERROR(WebService, "{} to {} returned null (httplib Error: {})", method, host + path,
+                      httplib::to_string(error));
             return WebResult{WebResult::Code::LibError, "Null response", ""};
         }