AWS: Add fallback support in ${cf} and ${s3}

[exoego]

What did you implement:

Add fallback support in ${cf} and ${s3}, so every variable syntaxes support fallback.

Closes #4940.
Closes #5756.

How did you implement it:

Currently, ${cf} and ${s3} will throw error if AWS request fails.
This behavior is supposed to be expected since it can prevent deploying broken service.
However, since current implementation of fallback (Variable#overwrite) requires all of Promises is fulfilled, ${cf} and ${s3} does not support fallback.

After this PR, #overwrite rid rejected promises, so first fulfilled value is used as fallback.

Note: I think this change makes it easier to address #3367, #5057, #5369 and #5757.

How can we verify it:

1. npm install -g exoego/serverless#default-values
2. sls deploy below and confirm it complete successfully.

service: aws-nodejs

provider:
  name: aws
  runtime: nodejs8.10

functions:
  hello:
    handler: handler.hello

custom:
  envVar: ${env:hoge, 'fallback'}
  optVar: ${opt:hoge, 'fallback'}
  selfVar: ${self:custom.xxx, 'fallback'}
  fileVar: ${file(./not-found):nooo, 'fallback'}
  cfVar: ${cf:no-such-stuck-exists.UNDEFINED_KEY, 'fallback'} 
  s3Var: ${s3:no/such/key, 'fallback'}
  ssmVar: ${ssm:/path/to/service/myParam, 'fallback'}

3. sls deploy below and confirm it aborts due to lack of fallback.

service: aws-nodejs

provider:
  name: aws
  runtime: nodejs8.10

functions:
  hello:
    handler: handler.hello

custom:
  envVar: ${env:hoge, 'fallback'}
  optVar: ${opt:hoge, 'fallback'}
  selfVar: ${self:custom.xxx, 'fallback'}
  fileVar: ${file(./not-found):nooo, 'fallback'}
  cfVar: ${cf:no-such-stuck-exists.UNDEFINED_KEY}  # no fallback !
  s3Var: ${s3:no/such/key} # no fallback !
  ssmVar: ${ssm:/path/to/service/myParam, 'fallback'}

Todos:

• Write tests
• Write documentation N/A
• Fix linting errors
• Make sure code coverage hasn't dropped
• Provide verification config / commands / resources
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

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

[eahefnawy]

@exoego with this change, if you reference invalid cf or s3 source without fallback, would you still get an error? or it'd just be caught and ignored?

[exoego]

@exoego with this change, if you reference invalid cf or s3 source without fallback, would you still get an error? or it'd just be caught and ignored?

It raises error if no fallback, as same as current.
There are already test codes for ${s3} and ${cf}.

Added the cases in How can we verify it too.

[eahefnawy]

Beautiful! Working on my end! Thanks @exoego 🙌

LG2M

[bpuertolas]

It doesn't work if the user is not logged or if the token included in the request is expired. Is it the desired behaviour ?