patch 8.2.3975: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move more error messages to errors.h.
diff --git a/src/fileio.c b/src/fileio.c
index 1aeb483..7c63a1a 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -4155,7 +4155,7 @@
 	    if (n)
 	    {
 		if (!bufref_valid(&bufref))
-		    emsg(_("E246: FileChangedShell autocommand deleted buffer"));
+		    emsg(_(e_filechangedshell_autocommand_deleted_buffer));
 #ifdef FEAT_EVAL
 		s = get_vim_var_str(VV_FCS_CHOICE);
 		if (STRCMP(s, "reload") == 0 && *reason != 'd')