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/window.c b/src/window.c
index 926a3f3..cc5f7df 100644
--- a/src/window.c
+++ b/src/window.c
@@ -1948,7 +1948,7 @@
     {
 	if (win1->w_frame->fr_parent != win2->w_frame->fr_parent)
 	{
-	    iemsg("INTERNAL: trying to move a window into another frame");
+	    iemsg("Trying to move a window into another frame");
 	    return;
 	}