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