Do not ignore init() errors (#2778)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2778

XnnpackBackend logged but did not return errors. This meant that the runtime could try to execute uninitialized delegate handles, leading to bad behavior like seg faults.

Now it returns the error to the runtime, failing clearly and gracefully during model load.

Reviewed By: digantdesai, mcr229

Differential Revision: D55549758

fbshipit-source-id: b76fbb72533f931a9a26cd280f76927c43e767d9