[lib] Remove errorHandler from createRealThreadFromPendingThread

Summary: In D718 we introduced thread creation hook which had a handleError parameter. When it was present, all exceptions would be caught and undefined was returned instead. This caused the returned promise to be optional and that was reflected in pendingThreadCreations map in input state containers. In D1140 and D1139 we removed all the handleError usages and now we can use Promise<string> instead which simplifies the code.

Test Plan: Flow, tested with other diffs

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: KatPo, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D1141