patch 9.0.1576: users may not know what to do with an internal error

Problem:    Users may not know what to do with an internal error.
Solution:   Add a translated message with instructions.
diff --git a/src/channel.c b/src/channel.c
index 97eb5db..5d45ac6 100644
--- a/src/channel.c
+++ b/src/channel.c
@@ -1757,7 +1757,7 @@
     typval_T	rettv;
 
     if (safe_to_invoke_callback == 0)
-	iemsg("INTERNAL: Invoking callback when it is not safe");
+	iemsg("Invoking callback when it is not safe");
 
     argv[0].v_type = VAR_CHANNEL;
     argv[0].vval.v_channel = channel;
@@ -2400,7 +2400,7 @@
 	    }
 	    return;
 	}
-    siemsg("INTERNAL: channel_remove_block_id: cannot find id %d", id);
+    siemsg("channel_remove_block_id(): cannot find id %d", id);
 }
 
 /*