Fix chunk upload fallback behavior

Summary:
Ref T7594. Currently, if a chunk upload fails, we incorrectly swallow the failure and fall back to single-file upload, which will often fail by hitting size limits. This also silences the original error.

Instead, do chunk uploads outside the block so that any exceptions escape, and we don't try to fall back to single-file upload.

Mostly just trying to get more info about what's going wrong on @joshuaspence's install.

Test Plan: Faked an exception in chunk upload, ran arc upload on a big file, saw the exception displayed on the console.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley, joshuaspence

Maniphest Tasks: T7594

Differential Revision: https://secure.phabricator.com/D12111