Make check_po.sh exit with 0 iff. the filtered i18nspector's output was empty.

It'll allow easier integration within our CI thingies.

This change has the drawback that we're buffering output, and displaying it only
once all i18nspector processes have completed. I say it's acceptable, now that
their accumulated runtime is quite low. There might be a way to do this better,
with some trick to capture stdout without buffering it.