Redirect PortgreSQL tools check output to stderr

Fixes a bug [found by Steven Yang](https://github.com/Kloadut/dokku-pg-plugin/pull/29#issuecomment-38789249) (@yangchenyun).

When the `pg_dump` executable is not found, the error message, instead of being shown on the terminal, were redirect to the file that is expected to contain only the database dump.