[TEST] when the test fail, have the exception message as the reason
the test failed but couldn't repro (yet), at the very least, make sure we have the exception message as the reason, can help to track down the failure itself when it happens again
Make sure we don't reuse arrays when sending and error back
We want to make sure recycling will not fail for any reason while trying to send a response back that is caused by a failure, for example, if we have circuit breaker on it (at one point), sending an error back will not be affected by it.
closes #6631
Thread pool rejection status code should be 429
Thread rejection should return too many requests status code, and not 503, which is used to also show that the cluster is not available
 relates to #6627, but only for rejections for now
closes #6629
[STORE] Wrap RateLimiter rather than copy RateLimitedIndexOutput

We clone RateLimitedIndexOutput from lucene just to collect pausing
statistics we can do this in a more straight forward way in a delegating
RateLimiter.

Closes #6625
[TEST] Change es.node.mode default for tests to `local`

In order to speed up test execution we should run in local mode by
default. CI builds will still use network builds all the time.

Closes #6624