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 in OPSTR.
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.