[web-db] change Error type to string

Summary:
Because of the Safari problems with structured clone algorithm we can not send objects with methods between threads.
Error objects has some methods inside core definition.

That was causing DataCloneError: The object could not be cloned..

Test Plan: Check is error are properly handled.

Reviewers: michal, tomek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D8116