Maxima: check if expression is defined

I want to test wether certain expressions are deifned, without having the programm to be stoped. For example, if have two matrices:

I know that the product (A*B) of these two is not well defined. So i would like to have maxima print out that this product is not defined without interrupting the evaluation of more expression that are to be tested.

Perhaps you are interested in the errcatch function that evaluates expressions and returns the results if there are no errors but returns an empty list if some error occurred.

You'll have to do a bit more work to do what you want, but at least Maxima won't stop on an error.