Use error on sender/receiver when reporting error on link closing

[ramya-rao-a]

Description

If the error event is fired on closing a link, we currently only report context.session.error as the error.
But, it can so happen that the error is actual on the sender/receiver based on the link type and there is no session error. We found this in Azure/azure-sdk-for-js#9615

This PR makes changes to surface errors on the sender/receiver and then falling back on the session instead of always surfacing the error on the session.

Reference to any github issues

• None

[ramya-rao-a]

cc @chradek because he is investigating the upstream issue

cc @grs to confirm that the error event fired when closing a sender or receiver link is accompanied by the actual error in the context.sender or context.receiver

[ramya-rao-a]

Tests added to sender.spec.ts and receiver.spec.ts files would fail before the fix in this PR.
Tests added to session.spec.ts is just for completion sake.

[chradek]

Looks good to me!