You can trap the error using INTRDEF 100, n, 1
where n is the custom function number (1 to 256) which is used to trap the run time error. All you need to do is write some code in the function #n where you want to handle the errors.
What is displayed on the LCD indicates the function number and the code address where the runtime error occur. You can then open that particular custom function and press F8 to get a list of the code addresses for that function, which you can then narrow down to find where the runtime error took place.