Drop unnecessary exception handler
Drop unnecessary exception handler

Summary:
The exception mentioned in the deleted comment is no longer possible after
D16939688 (https://github.com/facebook/flow/commit/9c04b09a5f4729da86a7b2546fca711c26eba538), so we can remove the error handler. As is, the hash_stats function
can not possibly raise an exception.

Reviewed By: nmote

Differential Revision: D25280254

fbshipit-source-id: cda353a3046a319c0167205a482fb9a8aa8023c9

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow