Streams: send an error to consumers blocked on non-existing group.

To detect when the group (or the whole key) is destroyed to send an
error to the consumers blocked in such group is a problem, so we leave
the consumers listening, the sysadmin is free to create or destroy
groups assuming she/he knows what to do. However a client may be blocked
in a given consumer group, that is later destroyed. Then the stream
receives new elements. In that case there is no sane behavior to serve
the consumer... but to report an error about the group no longer
existing.

More about detecting this synchronously and why it is not done:

1. Normally we don't do that, we leave clients blocked for other data
types such as lists.

2. When we free a stream object there is no longer information about
what was the key it was associated with, so while destroying the
consumer groups we miss the info to unblock the clients in that moment.

3. Objects can be reclaimed in other threads where it is no longer safe
to do client operations.

src/blocked.c

@@ -387,7 +387,11 @@ void handleClientsBlockedOnKeys(void) {
                             /* If the group was not found, send an error
                              * to the consumer. */
                             if (!group) {
-                                /* XXX: Fixme, send the error. */
+                                addReplyError(receiver,
+                                    "-NOGROUP the consumer group this client "
+                                    "was blocked on no longer exists");
+                                unblockClient(receiver);
+                                continue;
                             } else {
                                 *gt = group->last_id;
                             }