Change to GENIRESPONSE_FORBIDDEN when silver is not "ready" for a snapshot,

so that user sees the message (but not us). I feel like we should add a new
response code: GENIRESPONSE_NOTREADY.

protogeni/lib/GeniCMV2.pm.in

@@ -3043,7 +3043,7 @@ sub CreateImage($)
 				    "Could not compute status for sliver");
     }
     if ($status ne "ready") {
-	return GeniResponse->Create(GENIRESPONSE_SEARCHFAILED, undef,
+	return GeniResponse->Create(GENIRESPONSE_FORBIDDEN, undef,
 				    "sliver is not ready");
     }
     my $node_id = $sliver->resource_id();