Why should there be an error returned? There is no error in writing or reading. You are not doing an error query of the instrument.
If you were to use the driver, you would find the error query function and you could place that in your code to check to see if you have sent an illegal command. What would be better though, would be to ensure that the value that is entered into the string is valid before wasting the time with the VISA Write and Read. An empty string should never be allowed to exit the while loop.
