patch 8.2.3453: autocmd not executed when editing a directory

Problem:    Autocmd not executed when editing a directory ending in a path
            separator inside try block.
Solution:   Return NOTDONE instead of FAIL. (closes #8885)

src/fileio.c

@@ -338,7 +338,7 @@ readfile(
 	    filemess(curbuf, fname, (char_u *)_(msg_is_a_directory), 0);
 	    msg_end();
 	    msg_scroll = msg_save;
-	    return FAIL;
+	    return NOTDONE;
 	}
     }