set_color: Print an error for unknown options

This was forgotten, so e.g. calling `set_color --bg foo` results in
nothing being printed, which might result in strings being removed - #5443.

src/builtin_set_color.cpp

@@ -180,6 +180,7 @@ maybe_t<int> builtin_set_color(parser_t &parser, io_streams_t &streams, const wc
                 return STATUS_INVALID_ARGS;
             }
             case '?': {
+                builtin_unknown_option(parser, streams, L"set_color", argv[w.woptind - 1]);
                 return STATUS_INVALID_ARGS;
             }
             default: {