A = 0, always (call level, other values reserved for system use)
B = 0 (always local)
HL = address of new error handler
HL = 0, use default system error handler (in lower 8K of segment 0)
Fc = 0, always
A = old call level
HL = address of old error handler
Registers changed after return:
Make sure that the error handler is not paged out of the corresponding segment, when calling OZ. If an error occurs random information will be executed, since the original code of the particular bank has been paged out.
Setting B (in) can be discarded, OZ does not check nor used this value.
GN_Err, display an interactive error box
GN_Esp, return an extended pointer to a system error message
OS_Erc, get error context
OS_Esc, examine special condition