How to handle write/submit errors in eForm?

I am using eForm to write to an third party API, where errors like failed connection or invalid data can occur. I wrote a function to submit (write) to the API, which is called using eFormOnBeforeMailSent. The code works, but I need to handle the event of an error appropriately. How can I return the error message to the form at this point? Are there any examples for this? I am okay with it, if the example describes the error handling when writing to a database, file or something. I just need to know, how I can integrate error response handling in eForm.