patch 8.2.4003: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move more error messages to errors.h.
diff --git a/src/channel.c b/src/channel.c
index ec91739..3dea780 100644
--- a/src/channel.c
+++ b/src/channel.c
@@ -4051,7 +4051,7 @@
 	if (!channel->ch_error && fun != NULL)
 	{
 	    ch_error(channel, "%s(): write while not connected", fun);
-	    semsg(_("E630: %s(): write while not connected"), fun);
+	    semsg(_(e_str_write_while_not_connected), fun);
 	}
 	channel->ch_error = TRUE;
 	return FAIL;
@@ -4197,7 +4197,7 @@
 	    if (!channel->ch_error && fun != NULL)
 	    {
 		ch_error(channel, "%s(): write failed", fun);
-		semsg(_("E631: %s(): write failed"), fun);
+		semsg(_(e_str_write_failed), fun);
 	    }
 	    channel->ch_error = TRUE;
 	    return FAIL;