files: also when creating a backup fails, ask the user whether to proceed

When asking this question when deleting fails or writing fails,
it should be asked too when creating fails.  Otherwise the user
is blocked from saving the file -- until she realizes that maybe
toggling off backups would help.

src/files.c

@@ -1660,7 +1660,10 @@ bool write_file(const char *name, FILE *thefile, bool tmp,
 			backup_file = fdopen(backup_fd, "wb");
 
 		if (backup_file == NULL) {
-			statusline(HUSH, _("Error writing backup file %s: %s"),
+			warn_and_briefly_pause(_("Cannot create backup file"));
+			if (user_wants_to_proceed())
+				goto skip_backup;
+			statusline(HUSH, _("Cannot create backup %s: %s"),
 								backupname, strerror(errno));
 			free(backupname);
 			goto cleanup_and_exit;