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.
```
8d6bbaa85f