Incorrect error message when missing `services` field in metadata files.

[beqqrry-aws]

I received this error when creating a new scenario in the s3_metadata.yaml file:

DocGen Error: In /Users/beqqrry/PhpstormProjects/aws-doc-sdk-examples/.doc_gen/metadata/s3_metadata.yaml at s3_Scenario_ExpressBasics, example s3_Scenario_ExpressBasics name does not match the required format of 'svc_Operation', 'svc_Operation_Specialization', or 'cross_Title' (service s3, services )

However, the problem was that I was missing the services field.

Desired output: When the problem is a missing field, the error should indicate the missing field and not incorrectly assert that something unrelated is incorrect.