 The top bit of a return code is 0 if it is system dependent or 1 if it
     conforms to the following rules:

 The next two bits have the following significance:
        00  -  don't know about the nature of this return code
        01  -  error, but it isn't fatal, retry if appropriate
        10  -  fatal error, don't retry, caller is at fault
        11  -  fatal error, don't retry, called service is at fault

 The rest of the 16-bit return code (13 bits) are interpreted using the
     following list:

0000    failure, non specific
0001    failure, reply block contains text message
0002    unknown function
0003    congestion; system full (too many users)
0004    facility (on which service depends) temporarily not available
0005    invalid parameters
0006    unauthorised call
0007    internal error, try again later
0008    failure, reply block contains my system return code
0009    service does not exist (at time of request at least)
000A    service temporarily withdrawn
000B    unexpected request block size
000C    service's resource exhausted
004n    invalid parameter n




