x/playground: return non-200 for timeout errors

[toothrot]

Timeout errors currently must return a 200 status to be rendered correctly by the Playground javascript. The javascript needs to be updated to also parse 4xx errors correctly, then the playground should be updated to return responses with a correct code. Currently, unsuccessful requests of any kind are treated the same as a 5xx.

This may have implications for the Tour or other places where the playground is used.

Related to #38546

[gopherbot]

Change https://golang.org/cl/229307 mentions this issue: playground: stop storing timeouts in cache