patch 8.2.3983: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move more error messages to errors.h.
diff --git a/src/memfile.c b/src/memfile.c
index 4e8decb..b5ab2d9 100644
--- a/src/memfile.c
+++ b/src/memfile.c
@@ -1264,7 +1264,7 @@
     if ((flags & O_CREAT) && mch_lstat((char *)mfp->mf_fname, &sb) >= 0)
     {
 	mfp->mf_fd = -1;
-	emsg(_("E300: Swap file already exists (symlink attack?)"));
+	emsg(_(e_swap_file_already_exists_symlink_attack));
     }
     else
 #endif