Filter pastes at the query level if we're unable to load their content

Summary:
Fixes T8230. I think I broke this in D12354 since I missed the fact that this could filter results, because it's constructed in a sort of weird way. Try to make the logic a little more clear, while retaining the relevant properties:

• Pastes with unloadable content should be discarded;
• the input order should be retained in the result set.

Test Plan: Faked content misses and saw things work instead of fatal.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8230

Differential Revision: https://secure.phabricator.com/D12909