Fail in a more comprehensible way when a WorkingCopy lease omits or mangles "repositories.map"

Summary: Ref T13676. When the required "repositories.map" attribute is omitted, bin/drydock lease currently fatals in an unhelpful way when trying to lease a working copy.

Test Plan:
Ran bin/drydock lease --type working-copy with no attributes, after following steps in T13676.

<Allocation Failed> One or more blueprints promised a new resource, but failed when allocating: [PhutilAggregateException] All blueprints failed to allocate a suitable new resource when trying to allocate lease ("PHID-DRYL-orbtwtlinksm3xqpyhmw").
    - Exception: Working copy lease is missing required attribute "repositories.map".

      Attribute "repositories.map" should be a map of repository specifications.

Subscribers: yelirekim, PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13676

Differential Revision: https://secure.phabricator.com/D21796