patch 8.0.0160: EMSG() is sometimes used where it should be IEMSG()

Problem:    EMSG() is sometimes used for internal errors.
Solution:   Change them to IEMSG(). (Dominique Pelle)  And a few more.
diff --git a/src/channel.c b/src/channel.c
index 32f3527..f409e1d 100644
--- a/src/channel.c
+++ b/src/channel.c
@@ -1567,7 +1567,7 @@
     int		dummy;
 
     if (safe_to_invoke_callback == 0)
-	EMSG("INTERNAL: Invoking callback when it is not safe");
+	IEMSG("INTERNAL: Invoking callback when it is not safe");
 
     argv[0].v_type = VAR_CHANNEL;
     argv[0].vval.v_channel = channel;