ExecutionInterceptor access to request attempt exceptions

Maybe this is just a documentation change that is needed but reading through the `ExecutionInterceptor` comments and traversing through the code I don't see a way to get access to the intermediate exceptions that get retried.

## Expected Behavior
`ExecutionInterceptor` should have a hook like `onRequestFailure` that would give access to the exception for a specific HTTP request. This would provide the ability to see throttles and other types of events at the request level when it may resolve during the retries.

## Current Behavior
The current behavior as I understand it is to add some internal state to the retry handler but to only emit that once the retries have been exhausted.

## Possible Solution
Add a new Stage for `RequestFailure` that the `RetryableStage` can delegate to for these hooks

## Context
When we instrumented the V1 SDK we had the ability to retrieve the intermediate exceptions. With V2 that is not possible at the moment. See https://github.com/openzipkin/zipkin-aws/issues/116 for the corresponding issue on our end.

Beginne mit den Kommentaren zu ExecutionInterceptor und verfolge RetryableStage sowie den im Issue erwähnten retry handler. Ermittle, wo Zwischenfehler derzeit aufbewahrt oder ausgegeben werden, und überprüfe anschließend, dass Informationen zu Fehlern auf Anfrageebene während der Wiederholungsversuche verfügbar sind, während das bestehende Verhalten bei erschöpften Wiederholungsversuchen erhalten bleibt.

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