Unused values in enumeration classes

I have a large set of enumeration classes. I have been told the definition for one such class (mode) as:

 Init (34)
 Operating (10)
 Operating_NoInterrupts(18)

Sometimes, the data it applies to is invalid (stuck on an undefined value), or valid, but not specified (which looks the same). Eg.

 my_actual_mode_number = 50;
 mode(50) 
 Error using mode
 Cannot find a member of the 'mode' enumeration class that corresponds to each element of the given input argument.

Obviously, I would prefer it if it was able to return an unknown value enumeration, something like "unknown?50"

Is anyone aware of a way of achieving that within the enumeration class definition rather than the far more unwieldly method of essentially doing it myself, in main code?