psql dones't reflect exit status if input command via stdin

[magodo <wztdyl@xxxxxxxx>]

Hello,

I found when running command like `# echo "xxx" | psql postgres
postgres`, the return code is always 0 even though the command ("xxx")
here is of invalid syntax. While the `psql -c` way handled exit code
correctly.

In the meanwhile, mysql client handled both cases correctly.

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Zhaoting.Weng