Issue #24315:improve missing/duplicate po number message

The selects at lines 68 and 78 can be combined:
```SQL
select quhead_number, coalesce(quhead_custponumber, ''), cohead_id into ...
from quhead left outer join cohead on (...)
where quhead_id=pQuheadid;
```
If the `quhead_custponumber` is empty then raise one exception, if the `cohead_id` is not empty then raise the other.

Does anyone have an opinion on whether or not they _should_ be combined?