Emit cache setup issues from cache specs

Summary:
Ref T5501. Currently, we emit some bad warnings about, e.g., "apc.stat" on PHP 5.5+ systems with OPcache, where the warnings are not relevant.

Generate and raise warnings out of the CacheSpec pipeline so we only run relevant code.

Test Plan: Faked various warnings and saw them render correctly.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5501

Differential Revision: https://secure.phabricator.com/D12318