Throw a more tailored exception
Throw a more tailored exception

Summary: Throw a `PhutilProxyException` instead of a plain old `Exception`.

Test Plan: N/A

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11518

julkiewicz/libphutil,julkiewicz/libphutil,julkiewicz/libphutil