loop: instruction the agent not to make new branches

Things always get muddled when it starts making lots of branches.
diff --git a/loop/agent.go b/loop/agent.go
index cc56f13..fbd759f 100644
--- a/loop/agent.go
+++ b/loop/agent.go
@@ -1106,7 +1106,7 @@
 
 			b := a.BranchName()
 			if b != "" {
-				return nil, fmt.Errorf("branch already set to: %s", b)
+				return nil, fmt.Errorf("branch already set to %s; do not create a new branch", b)
 			}
 
 			if params.BranchName == "" {