Fix excessively harsh validation of certain complex configuration

Summary:
See IRC. We're supposed to repair configuration, but if custom validators throw a generic Exception or use PhutilTypeSpec to do a check, we may explode way harder than we intend to.

Instead, soften these exceptions into validation exceptions so we repair configuration, raise a setup issue, and continue.

Test Plan:

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D14998