If PQcancel() fails, consider it as if the master is failing.

Because PQcancel() establish a new synchronous connection to the
database, if it fails it means something wrong has happenned with
master. So instead of just ignore the failure, CancelQuery() now
reports a failure condition so we can detect master's death in
that situation.

This is very important specially when only postmaster crashes but
other children/backend connections are still there. Because the
children connection won't fail and CancelQuery() failure is our
only indication of something wrong happenning.
Currently we just ignore the PQcancel() failure which leads us to
a situation in which we just loop forever
trying to cancel the async query.

Reported by: Martin Euser <martin.euser@nl.abnamro.com>
Problem analyzed and bug spotted by: Andres Freund <andres@2ndquadrant.com>
Patch by: Jaime Casanova <jaime@2ndquadrant.com>