Cherry-pick of #16631 to RC4.0.0_master

Purpose

Addresses CORE: Stuck when cancelling publishing a package in Package Manager. The issue was that when cancelling out of the folder selection dialog, we weren't notifying the frontend client.

Changes

Declarations

Check these if you believe they are true

• Is documented according to the standards
• The level of testing this PR includes is appropriate
• Changes to the API follow Semantic Versioning and are documented in the API Changes document.

Release Notes

• the backend was not notifying the web client when cancelling out of the folder selection dialog during the publish process
• now correctly sets the Uploading back to False, and notifies the client
• added test for this flow

Reviewers

@QilongTang

FYIs

@zeusongit
@jnealb