get_or_create_ref in json validator uses exceptions for flow control

[BalrogOfHell]

The following try/catch block exists in the get_or_create_ref function:

                         try {
				auto &subschema = file.unknown_keywords.at(uri.pointer()); // null is returned if not existing
				auto s = schema::make(subschema, this, {}, {{uri}});       //  A JSON Schema MUST be an object or a boolean.
				if (s) {                                                   // nullptr if invalid schema, e.g. null
					file.unknown_keywords.erase(uri.fragment());
					return s;
				}
			} catch (nlohmann::detail::out_of_range &) { // at() did not find it
			}

An out_of_range exception from nlohmann::json::at() is thrown when there is no schema to be found with the given JSON pointer, and this seems to be expected. It is generally frowned upon to use exceptions like this, ref isocpp. I would like to suggest the following fix using nlohmann::json::find instead:

   auto subschema_it = file.unknown_keywords.find(uri.pointer());
   if (subschema_it != file.unknown_keywords.end()) { // if found
      auto &subschema = subschema_it->second;
      auto s = schema::make(subschema, this, {}, {{uri}});
      if (s) { // if schema is valid (non-null)
         file.unknown_keywords.erase(uri.fragment());
         return s;
      }
   }

Noted this issue as our repository started to use more json references, this exception ends up being thrown a lot.

[pboettch]

Good point. Could you provide a PR?

[BalrogOfHell]

Yessir!
#334

[BalrogOfHell]

@pboettch Unsure how to debug or re-run the CI builds failing on that PR. Does not seem like I have the ability to re-trigger them. May need assistance there! 🙏