Change/fix some cluster script error reporting.

The main script now uses the Reporter::error BIF instead of scheduling
a reporter_error event to report an invalid node name.  The later only
works if the reporter framework is loaded, but the BIF will do the right
thing and use stderr if there's no event handler.

I also @if'd out most of the setup-connections script when the node is
invalid because that's what the cluster/__load__.bro would normally do.

scripts/base/frameworks/cluster/setup-connections.bro

@@ -1,6 +1,8 @@
 @load ./main
 @load base/frameworks/communication/main
 
+@if ( Cluster::node in Cluster::nodes )
+
 module Cluster;
 
 event bro_init() &priority=9
@@ -80,3 +82,5 @@ event bro_init() &priority=9
 			}
 		}
 	}
+
+@endif