[keyserver] Don't crash on malformed socket input

Summary:
This diff makes it so now if checkInputValidator throws a ServerError due to input validation failing, clientSocketMessage will be set in this catch block.

This is important because the catch block has an invariant expecting the clientSocketMessage to be set.

Test Plan: I had a reproducible issue where I could trigger checkInputValidator to throw a ServerError. I confirmed that the invariant was no longer triggered after my change

Reviewers: tomek, atul

Reviewed By: atul

Differential Revision: https://phab.comm.dev/D6370