patch 8.2.3977: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move more error messages to errors.h.
diff --git a/src/textprop.c b/src/textprop.c
index 4c17a4a..c28d4e4 100644
--- a/src/textprop.c
+++ b/src/textprop.c
@@ -222,7 +222,7 @@
 
     if (buf->b_ml.ml_mfp == NULL)
     {
-	emsg(_("E275: Cannot add text property to unloaded buffer"));
+	emsg(_(e_cannot_add_text_property_to_unloaded_buffer));
 	return FAIL;
     }