Clarify RpcServer shutdown error for corner case

If the service tries to shutdown before join(), we end up in this case.

Test: none
Bug: none
Change-Id: I0d2f59b0d398ce3a18b522d7830417220b1a5145
diff --git a/libs/binder/RpcServer.cpp b/libs/binder/RpcServer.cpp
index 0d06e9e..851b407 100644
--- a/libs/binder/RpcServer.cpp
+++ b/libs/binder/RpcServer.cpp
@@ -295,7 +295,8 @@
 bool RpcServer::shutdown() {
     RpcMutexUniqueLock _l(mLock);
     if (mShutdownTrigger == nullptr) {
-        LOG_RPC_DETAIL("Cannot shutdown. No shutdown trigger installed (already shutdown?)");
+        LOG_RPC_DETAIL("Cannot shutdown. No shutdown trigger installed (already shutdown, or not "
+                       "joined yet?)");
         return false;
     }