Add test coverage for PhutilErrorHandler and silence operator ("@")

Summary:
Fixes T3351. The situation is actually less dire than the comment implies, I had slightly misrembered context. Basically:

• PhutilErrorHandler respects the silence operator ("@"); I'm not sure if it did at the time.
• Part of the issue was stack traces for unrelated things, but PhutilOpaqueEnvelope solved that.

Test Plan:

• Added test coverage for the beahvior of "@".
• Hit LDAP exceptions and verified nothing sensitive appeared in the traces/logs.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3351

Differential Revision: https://secure.phabricator.com/D6390