fix(layers): show warning if !Sub function is used in Layers section of template.yaml (#948)

[goldeneggg]

#948

Description of changes:

If !Sub function is used in Layers section of template.yaml and run sam local invoke and get an error.

• got: Error reason is not clear
• expected: We want to know more information by log

Resources:
  HelloWorldFunction:
    Type: AWS::Serverless::Function
    Properties:
      CodeUri: hello_world/
      Handler: app.lambdaHandler
      Runtime: nodejs8.10
      Events:
        HelloWorld:
          Type: Api
          Properties:
            Path: /hello
            Method: post
      Layers:
        - !Sub arn:aws:lambda:${AWS::Region}:123456789012:layer:MyLayer:1

$ sam local invoke -e event.json HelloWorldFunction

# This warning log is added by this pull request
2019-01-20 00:56:43 '!Sub' keyword in Layer configuration does not work locally. Please replace all parameter strings(like '${AWS::Region}') with the original value(like 'ap-northeast-1') and remove '!Sub' keyword.

Unable to import module 'app': Error

Checklist:

• Write Design Document (Do I need to write a design document?)
• Write unit tests
• Write/update functional tests
• Write/update integration tests
• make pr passes
• Write documentation

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[mnapoli]

Stumbled on that as well, until such feature is supported it would be awesome to have a clear warning 👍

[jfuss]

While this impacts Layers (probably disproportionally), I don't believe this is the correct place for adding this detail. SAM CLI (as a whole) does not have support for Sub (and many other intrinsics). Due to this, we shouldn't be trying to read these intrinsics within the higher provider classes. This means, for ever resource and property, we need to add this kind of logging. While it is contextually, it creates more work later on. Instead, I would be ok with adding this logging more broadly to communicate the known Intrinsic gaps. Maybe this is a good place?

[jfuss]

@goldeneggg and @mnapoli We are addressing intrinsics in #1261. Once that is merged, integrated, and released, there shouldn't be any more intrinsic confusion on support. I am going to close this in favor of the full solution we are currently going through.