In serverless@3 serverless.yml aws requests halt execution, even when a defaultValue is specified

Are you certain it's a bug?

• Yes, it looks like a bug

Is the issue caused by a plugin?

• It is not a plugin issue

Are you using the latest v3 release?

• Yes, I'm using the latest v3 release

Is there an existing issue for this?

• I have searched existing issues, it hasn't been reported yet

Issue description

When upgrading a project from serverless@2 to serverless@3, I'm noticing that there is a breaking change where any AWS related errors in serverless.yml immediately halt execution, even if a defaultValue is specified.

For example, in serverless@2 this worked fine:

SOME_KEY: ${ssm:/somepath/somekey~false, env:SOME_KEY}

Explanation: If we tried to fetch a value from AWS SSM, if the key didn't exist or the application did not have access, we could fall back to the defaultValue, in this case the environment variable SOME_KEY.

In serverless@3, this behavior has changed. The defaultValue is no longer usable. Execution immediately stops if there is an AWS error.

NOTE: At first this was reported in the serverless-offline plugin, here: https://github.com/dherault/serverless-offline/issues/1278 but dnalborczyk said:

the variable resolution is done by serverless itself, which in turn also throws the error. I don't think there's much we can do about it.

... so I am reporting it here.

Service configuration (serverless.yml) content

SOME_KEY: ${ssm:/somepath/somekey~false, env:SOME_KEY}

Command name and used flags

serverless whatever

Command output

Error:
Cannot resolve serverless.yml: Variables resolution errored with:
  - Cannot resolve variable at "provider.environment.SOME_KEY": AWS provider credentials not found. Learn how to set up AWS provider credentials in our docs here: <http://slss.io/aws-creds-setup>.

Environment information

Framework Core: 3.21.0 (local)
Plugin: 6.2.2
SDK: 4.3.2

Reproduce the failure with the serverless.yml configuration and the serverless whatever command, using the reported Framework Core 3.21.0 environment. Trace the variable-resolution entry point responsible for provider.environment.SOME_KEY; done means an AWS SSM error or missing credentials no longer prevents the specified defaultValue fallback from being used.