Force READY portals into FAILED state when a transaction or subtransaction

is aborted, if they were created within the failed xact.  This prevents
ExecutorEnd from being run on them, which is a good idea because they may
contain references to tables or other objects that no longer exist.
In particular this is hazardous when auto_explain is active, but it's
really rather surprising that nobody has seen an issue with this before.
I'm back-patching this to 8.4, since that's the first version that contains
auto_explain or an ExecutorEnd hook, but I wonder whether we shouldn't
back-patch further.