How to catch form submission errors using JavaScript API

Doesn't anyone know how to catch marketo submission errors using the JavaScript API, when `save2` does not POST?

We are submitting our Marketo forms using the JS API, and are having issues with payload not actually being delivered to Marketo. I've found that the `MktoForms2` object is loading correctly, but in some instances in our production environment, the `submit()` and `onSuccess()` methods are not executing. I've tentatively arrived at the conclusion that there is some kind of error. I can tell the difference because I'm not seeing a POST call to `index.php/leadCapture/save2` in Network. So I want to know why this payload doesn't being delivered!

What's worse is this issue only appears to be happening in our production environment, and not in out staging or dev. So naturally it's baffling that the same code would execute differently.

Here is the model function for submitting in the background:

proto.marketoPayload =  function (marketoPayload) {

   return new Promise( (resolve, reject)=>{

      //checks to make sure the Payload exists and that there is a marketo form.
      // The marketo script will always load, but sometimes here are no actual forms
      if( _model.empty( marketoPayload ) ||  _model.empty( MktoForms2.allForms() )  ){

         //define the error message for email
         const marketoError = `marketo error, NO PAYLOAD: ${_model.empty( marketoPayload)}, EXECUTION FAILURE: ${_model.empty( MktoForms2.allForms() ) } `;

 //send error message from the function
 console.log( marketoError );

 //update the payload
 APIObj.wp_email_subject = "Failed Marketo Submission";
 APIObj.error_message = marketoError;
 APIObj.wp_email_message = JSON.stringify(APIObj, null, 4);

 //send Fail email
 var sendEmail = _model.model__handleEmailSubmission(marketoPayload)
 .always((response) => {
 console.log('marketo fail ', response);

 });

 //exit this promise
 reject();
 }

 //marketo exists and is working, let's actually deliver the payload
 MktoForms2.whenReady( (form )=>{

 this.debug( 'Payload Success:', marketoPayload);
 this.debug( 'this is the form', form );

 form.addHiddenFields(marketoPayload);

 form.onSuccess( (vals)=> {

 console.log( 'Marketo Success: ', vals );

 return false;

 });

 form.submit();

 APIObj.wp_email_message = JSON.stringify(marketoPayload, null, 4);
 //send success email
 var sendEmail = _model.model__handleEmailSubmission(marketoPayload)
 .always((response) => {
 console.log('email response ', response);
 });

 resolve();

 } ); //end callback


 } ); //end promise


};