[native] improve error handling in parseOLMPrekeys

Summary:
addressing feedback from D11312

More info here: https://linear.app/comm/issue/ENG-7280/add-checks-to-parseolmprekey

Test Plan: tested both validateAndGet and validateAndUpload and confirmed both still work as expected. Also passed invalid prekey blobs to parseOLMPrekey to trigger the various exceptions there.

Reviewers: marcin

Reviewed By: marcin

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11356