Pass exception to PhutilProxyException
Summary: Fixes T12243. That error occured due to network flakiness with some mounted filesystems so I'm not sure how best to simulate it. But you can look and see that the PhutilProxyException does indeed expect an exception as its second arg.
Test Plan: Look at method signature... look at callsite... now back at the method. Smile and nod.
Reviewers: Blessed Reviewers, yelirekim, epriestley
Reviewed By: Blessed Reviewers, epriestley
Maniphest Tasks: T12243
Differential Revision: https://secure.phabricator.com/D17335