[lib] Add log and continue in getPushUserInfo when thread missing from ThreadStore

Summary:
This has come up twice now: ENG-9454 and ENG-9946.

I think it will help us in the future to have a more specific error message. It also is probably a good idea to continue; here, in case there are other notifs that we can send successfully.

Test Plan: Ran the code and confirmed the log was printed

Reviewers: angelika, kamil, tomek

Reviewed By: kamil

Differential Revision: https://phab.comm.dev/D14160