Don't be so trusting that shm_toc_lookup() will always succeed.

Given the possibility of race conditions and so on, it seems entirely
unsafe to just assume that shm_toc_lookup() always finds the key it's
looking for --- but that was exactly what all but one call site were
doing.  To fix, add a "bool noError" argument, similarly to what we
have in many other functions, and throw an error on an unexpected
lookup failure.  Remove now-redundant Asserts that a rather random
subset of call sites had.

I doubt this will throw any light on buildfarm member lorikeet's
recent failures, because if an unnoticed lookup failure were involved,
you'd kind of expect a null-pointer-dereference crash rather than the
observed symptom.  But you never know ... and this is better coding
practice even if it never catches anything.

Discussion: https://postgr.es/m/9697.1496675981@sss.pgh.pa.us

src/backend/executor/nodeForeignscan.c

@@ -344,7 +344,7 @@ ExecForeignScanInitializeWorker(ForeignScanState *node, shm_toc *toc)
 		int			plan_node_id = node->ss.ps.plan->plan_node_id;
 		void	   *coordinate;
 
-		coordinate = shm_toc_lookup(toc, plan_node_id);
+		coordinate = shm_toc_lookup(toc, plan_node_id, false);
 		fdwroutine->InitializeWorkerForeignScan(node, toc, coordinate);
 	}
 }