Minor cleanup for access/transam/parallel.c.

 Minor cleanup for access/transam/parallel.c.
 
 ParallelMessagePending *must* be marked volatile, because it's set
 by a signal handler.  On the other hand, it's pointless for
 HandleParallelMessageInterrupt to save/restore errno; that must be,
 and is, done at the outer level of the SIGUSR1 signal handler.
 
 Calling CHECK_FOR_INTERRUPTS() inside HandleParallelMessages, which itself
 is called from CHECK_FOR_INTERRUPTS(), seems both useless and hazardous.
 The comment claiming that this is needed to handle the error queue going
 away is certainly misguided, in any case.
 
 Improve a couple of error message texts, and use
 ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE to report loss of parallel worker
 connection, since that's what's used in e.g. tqueue.c.  (Maybe it would be
 worth inventing a dedicated ERRCODE for this type of failure?  But I do not
 think ERRCODE_INTERNAL_ERROR is appropriate.)
 
 Minor stylistic cleanups.