Fix error status of vacuumdb when multiple jobs are used

When running a batch of VACUUM or ANALYZE commands on a given database,
there were cases where it is possible to have vacuumdb not report an
error where it actually should, leading to incorrect status results.

Author: Julien Rouhaud
Reviewed-by: Amit Kapila, Michael Paquier
Discussion: https://postgr.es/m/CAOBaU_ZuTwz7CtqLYJ1Ouuh272bTQPLN8b1bAPk0bCBm4PDMTQ@mail.gmail.com
Backpatch-through: 9.5