TEXT and DATA I/O
These add-ons need to know how to behave in case of error. Let's say you are trying to open a file called FILE.TXT through the TEXT add-on. Couldn't the channel identifier retrieve the reading status, for example be set to -1 if the file is not found? Most of all, it would be needed that the rest of the GDL scripts be interpreted even if the add-on cannot find the file...
IF ch=-1 THEN
PRINT "file not found, but scripts will be interpeted all the same"