[Validator] Unable to find license file in sub specs after failing required license.

[kylef]

Steps to reproduce:

• Create a podspec without a license attribute
• Run pod lib lint (running the Core's validator) on the newly created spec

Actual result:

$ pod lib lint | grep license
- WARN  | Missing required attribute `license`.
- WARN  | [SpecName/Core, SpecName/List] Unable to find a license file

Expected result:

$ pod lib lint | grep license
- WARN  | Missing required attribute `license`.

Also, perhaps license file should apply to just the spec not sub spec since it's a single source repository?