Automatic merge from submit-queue

Timeout and Max-in-flight don't report non-resource URLs correctly.

Unify error reporting for 429 and 504 to be correct for timeout and max in flight and eviction. Add better messages to eviction (removing a todo). Return the correct body content for timeouts (reason and code should be correct).

This potentially increases cardinality of 429, but because non-api urls may be under the max-inflight budget we need to report them somewhere (if something breaks and starts fetching API versions endlessly).

```release-note
The 504 timeout error was returning a JSON error body that indicated it was a 500.  The body contents now correctly report a 500 error.
```