Add validation for global vars

[heyealex]

Add another validation stage for global vars where we validate that:

• No vars are set to null (unset in YAML)
• project_id is a global var (only warn if not)
• labels are the correct type (as they are needed elsewhere)

In addition, a few minor changes where made in validate.go in handling
errors to keep consistency.

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• Are all tests passing? make tests
• If applicable, have you written additional unit tests to cover this
  change?
• Is unit test coverage still above 80%?
• Have you updated any application documentation such as READMEs and user
  guides?
• Have you followed the guidelines in our Contributing document?