Don't reject when unmounting in `Zoomable`
Don't reject when unmounting in `Zoomable`

Summary:
This is causing exercises to fail to load in the native apps. In some cases, we seem to be
unmounting these components rather quickly--before the fonts have loaded--which leads to a Promise
rejection and an exercise kickout.

Test Plan:
- Open up an exercise in the native apps that was previously failing (e.g., `Systems of equations with elimination`).
    - Verify that it loads successfully.

Reviewers: benkomalo

Subscribers: jared, kevinb

Differential Revision: https://phabricator.khanacademy.org/D27920

ariabuckles/perseus,ariabuckles/perseus,learningequality/perseus,ariabuckles/perseus,ariabuckles/perseus,learningequality/perseus