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

Subscribers: epriestley

Maniphest Tasks: T12243

Differential Revision: https://secure.phabricator.com/D17335