Raise a better error when a file upload fails in Differential

Summary:
Ref T2296. If a file upload fails (e.g., too large), we read the textarea from the "Create New Diff" interface. This means we show the user an error like "empty diff" when we should show them an error like "file upload failed, patch is too large". This is part of the issue in T2296, which features a 2.5MB diff.

Instead, check if a file was specified, so we'll raise a better error.

Test Plan: Tried to upload a large patch, got a "file is too large" error instead of an empty-diff-related error.

Reviewers: btrahan, vrana, codeblock

Reviewed By: codeblock

CC: aran

Maniphest Tasks: T2296

Differential Revision: https://secure.phabricator.com/D4369