Fix region error handling in Lambda@Edge implementation

[pmuens]

What did you implement

Fixes a problem where the ServerlessError class was not used correctly. Also added the unit tests which checks if the region is validated.

How can we verify it

Run serverless package with the following serverless.yml. You shouldn't see an error due to a ServerlessError misuse (but you should see the error that the region is incorrect).

service: test-${self:custom.idx}

provider:
  name: aws
  runtime: nodejs10.x
  versionFunctions: false
  region: eu-central-1
  stage: dev

custom:
  idx: 0

functions:
  hello:
    handler: functions/handler.handler
    events:
      - cloudFront:
          eventType: viewer-response
          origin: https://serverless.com/framework/docs

Todos

Useful Scripts

• npm run test-ci --> Run all validation checks on proposed changes
• npm run lint-updated --> Lint all the updated files
• npm run lint:fix --> Automatically fix lint problems (if possible)
• npm run prettier-check-updated --> Check if updated files adhere to Prettier config
• npm run prettify-updated --> Prettify all the updated files

• Write and run all tests
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES
Is it a breaking change?: NO

[medikoo]

LGTM!

[pmuens]

@medikoo Thanks! 💪