Revert "removed piping of stderr in collect targets"
Revert "removed piping of stderr in collect targets"

Summary:
This reverts commit 9a4b66cb62f2364c1df516349d705c0c773c1c4b.
Turns out that piping of stderr is necessary, because we want the output to be a JSON. The correct fix would be to only print out stderr if there is an error, but since we are moving this logic to Changes client, let's not spend more time working on this.

Test Plan: N/A

Reviewers: anupc

Reviewed By: anupc

Subscribers: kylec

Differential Revision: https://tails.corp.dropbox.com/D225079

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