The rejected execution handler API says that rejectedExecution(Runnable,
ThreadPoolExecutor) throws a RejectedExecutionException if the task must
be rejected due to capacity on the executor. We do throw something that
smells like a RejectedExecutionException (it is named
EsRejectedExecutionException) yet we violate the API because
EsRejectedExecutionException is not a RejectedExecutionException. This
has caused problems before where we try to catch RejectedExecution when
invoking rejectedExecution but this causes EsRejectedExecutionException
to go uncaught. This commit addresses this by modifying
EsRejectedExecutionException to extend
RejectedExecutionException.
* Factor UnknownNamedObjectException into its own class
This moves the inner class `UnknownNamedObjectException` from
`NamedXContentRegistry` into a top-level class. This is so that
`NamedXContentRegistry` doesn't have to depend on StreamInput and StreamOutput.
Relates to #28504
