after et_root_schema, throw if there are undef refs

fix #97

src/json-validator.cpp

@@ -239,6 +239,12 @@ public:
 			if (!new_schema_loaded) // if no new schema loaded, no need to try again
 				break;
 		} while (1);
+
+		for (const auto &file : files_)
+			if (file.second.unresolved.size() != 0)
+				throw std::invalid_argument("after all files have been parsed, '" +
+				                            (file.first == "" ? "<root>" : file.first) +
+				                            "' has still undefined references.");
 	}
 
 	void validate(const json::json_pointer &ptr, const json &instance, json_patch &patch, error_handler &e) const final

test/issue-98.cpp

@@ -4,7 +4,12 @@ int main(void)
 {
 	nlohmann::json nlBase{{"$ref", "#/unknown/keywords"}};
 	nlohmann::json_schema::json_validator validator;
-	validator.set_root_schema(nlBase); // this line will log the caught exception
 
-	return 0;
+	try {
+		validator.set_root_schema(nlBase); // this line will log the caught exception
+	} catch (const std::exception &e) {
+		if (std::string("after all files have been parsed, '<root>' has still undefined references.") == e.what())
+			return EXIT_SUCCESS;
+	}
+	return EXIT_FAILURE;
 }