Replace all "attach first..." exceptions with `assertAttached()`

Summary:
Ref T3599
Go through everything, grep a bit, replace some bits.

Test Plan: Navigate around a bit

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran

Maniphest Tasks: T3599

Differential Revision: https://secure.phabricator.com/D6871