On the first call of this routine, the error table for the current
error context is copied into a holding area, the current context
is annulled and the first message in the holding area is returned.
Thereafter, each time the routine is called, the next message from
the holding area is returned.
The argument PARAM is the returned message name and PARLEN the length
of the message name.
OPSTR is the returned error message text and OPLEN is the length of
the error message.
If the message text is longer than the declared length of OPSTR, then the
message is truncated with an ellipsis, i.e. ``...'', but no
error results.
The status associated with the returned message is returned in STATUS
until there are no more messages to return - then STATUS is set to
SAI__OK, PARAM and OPSTR are set to blanks and PARLEN and OPLEN to 1.
If there are no messages pending on the first call, a warning message
is generated and returned with STATUS set to EMS__NOMSG.
After STATUS has been returned SAI__OK, the whole process is repeated
for subsequent calls.