Update ember-compatibility-functions to always return error codes i…

…nstead of directly encoding them as success. (#34392)

* Let the read implementation return the error status for failed reads.

The checkpoint logic and error statuses are duplicated between
Engine.cpp and ember-compatibility-functions.

This change makes the code consistent either error out (with
something that the engine can handle) or encode success.

* Fix typo

* One more code cleanup for return values. No more global statuses

* minor update to kick ci

* Update src/app/util/ember-compatibility-functions.cpp

Co-authored-by: Boris Zbarsky <bzbarsky@apple.com>

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Co-authored-by: Andrei Litvin <andreilitvin@google.com>
Co-authored-by: Boris Zbarsky <bzbarsky@apple.com>