patch 8.2.3069: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move some error messages to errors.h.  Use clearer names.
diff --git a/src/channel.c b/src/channel.c
index 2a93e25..5818dc3 100644
--- a/src/channel.c
+++ b/src/channel.c
@@ -1232,7 +1232,7 @@
 
 	    if (!buf->b_p_ma && !channel->ch_part[PART_OUT].ch_nomodifiable)
 	    {
-		emsg(_(e_modifiable));
+		emsg(_(e_cannot_make_changes_modifiable_is_off));
 	    }
 	    else
 	    {
@@ -1278,7 +1278,7 @@
 						!opt->jo_modifiable[PART_ERR];
 	    if (!buf->b_p_ma && !channel->ch_part[PART_ERR].ch_nomodifiable)
 	    {
-		emsg(_(e_modifiable));
+		emsg(_(e_cannot_make_changes_modifiable_is_off));
 	    }
 	    else
 	    {