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/ops.c b/src/ops.c
index 73faa16..8f2ca8e 100644
--- a/src/ops.c
+++ b/src/ops.c
@@ -621,7 +621,7 @@
 
     if (!curbuf->b_p_ma)
     {
-	emsg(_(e_modifiable));
+	emsg(_(e_cannot_make_changes_modifiable_is_off));
 	return FAIL;
     }