Implement Codec Blacklisting on Web Testpage

In some cases, a codec looks like it is supported web-side, but when decoding is actually tried with a frame, the encoder can fail.

Specifically, this is seen with `vp9` with `yuv444` format, where some browsers cannot process `yuv444` but will happily act like they can. They are in fact fine with `yuv442` format, which is next to be tried in the negotiation process.

However, this negotiation doesn't happen because the connection is killed when the decode fails. We should make the web-side remember what codec failed before reconnecting and trying again.