psql: call clearerr() just before printing
We were never doing clearerr() on the output stream, which results in a
message being printed after each result once an EOF is seen:
could not print result table: Success
This message was added by commit
b03436994bcc (in the pg13 era); before
that, the error indicator would never be examined. So backpatch only
that far back, even though the actual bug (to wit: the fact that the
error indicator is never cleared) is older.