patch 8.2.3750: error messages are everywhere

Problem:    Error messages are everywhere.
Solution:   Move more error messages to errors.h and adjust the names.
diff --git a/src/channel.c b/src/channel.c
index 7514d63..6343124 100644
--- a/src/channel.c
+++ b/src/channel.c
@@ -1210,7 +1210,8 @@
 	{
 	    buf = buflist_findnr(opt->jo_io_buf[PART_OUT]);
 	    if (buf == NULL)
-		semsg(_(e_nobufnr), (long)opt->jo_io_buf[PART_OUT]);
+		semsg(_(e_buffer_nr_does_not_exist),
+					       (long)opt->jo_io_buf[PART_OUT]);
 	}
 	else
 	{
@@ -1257,7 +1258,8 @@
 	{
 	    buf = buflist_findnr(opt->jo_io_buf[PART_ERR]);
 	    if (buf == NULL)
-		semsg(_(e_nobufnr), (long)opt->jo_io_buf[PART_ERR]);
+		semsg(_(e_buffer_nr_does_not_exist),
+					       (long)opt->jo_io_buf[PART_ERR]);
 	}
 	else
 	{