How to catch default error when invalid datetime format entered in dateTimeField?

You cannot capture the message. If you want, you can create it using a combination of two textFields() that will give you access to its value if want to validate, as in the current scenario, you will not be able to access the value of it, It will only store the value when no validation is triggered. 

The order is 

1. You enter the value. 
2. Components checks the value and see if it follows all the applied validations. 
3. If yes, then the value gets saved to the specified variable (local or ri)

Now you either check the value for null or if you really want the value, then replace it with a textField!

You cannot capture the message. If you want, you can create it using a combination of two textFields() that will give you access to its value if want to validate, as in the current scenario, you will not be able to access the value of it, It will only store the value when no validation is triggered. 

The order is 

1. You enter the value. 
2. Components checks the value and see if it follows all the applied validations. 
3. If yes, then the value gets saved to the specified variable (local or ri)

Now you either check the value for null or if you really want the value, then replace it with a textField!