If repository mirroring fails, keep trying the other mirrors

Summary: Ref T4338. Currently, if you have several mirrors and the first one fails, we won't try the other mirrors (since we'll throw and that will take us out of the mirroring process). Instead, try each mirror even if one fails, and then throw an AggregateException with all the failures.

Test Plan:
  - Ran `bin/repository mirror` normally.
  - Faked an exception, ran again, got the AggregateException I expected.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4338

Differential Revision: https://secure.phabricator.com/D8067