fix(cts): friendly error message when test is missing

Author: millotp

generators/src/main/java/com/algolia/codegen/cts/tests/TestsRequest.java

@@ -54,7 +54,16 @@ public void run(Map<String, CodegenModel> models, Map<String, CodegenOperation>
     for (Map.Entry<String, CodegenOperation> entry : operations.entrySet()) {
       String operationId = entry.getKey();
       if (!cts.containsKey(operationId)) {
-        throw new CTSException("operationId " + operationId + " does not exist in the spec");
+        throw new CTSException(
+          "operationId '" +
+          operationId +
+          "' does not exist in the tests suite, please create the file:" +
+          " 'tests/CTS/methods/requests/" +
+          client +
+          "/" +
+          operationId +
+          ".json'"
+        );
       }
       Request[] op = cts.get(operationId);