[SPARK-21253][CORE] Fix a bug that StreamCallback may not be notified if network errors happen

## What changes were proposed in this pull request?

If a network error happens before processing StreamResponse/StreamFailure events, StreamCallback.onFailure won't be called.

This PR fixes `failOutstandingRequests` to also notify outstanding StreamCallbacks.

## How was this patch tested?

The new unit tests.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #18472 from zsxwing/fix-stream-2.