[Bug]: Copilot env deploy shows null error instead of actual CloudFormation error message

[fuzzbuzzz]

Description:
When deploying an environment addon using copilot env deploy, CloudFormation errors are not properly surfaced to the user. Instead of showing the actual error message, Copilot displays a generic "null" error message.

Steps to Reproduce:

1. Create a Copilot application with an environment
2. Add an addon template (CloudFormation) with a deliberate error (e.g., invalid RDS engine version)
3. Run copilot env deploy

Actual Result:
Error message displayed:
"Embedded stack arn:aws:cloudformation:[region]:[account]:stack/[stack-name] was not successfully created: null"

Expected Result:
The actual CloudFormation error message should be displayed, such as:
"Resource handler returned message: Cannot find version 14.7 for postgres (Service: Rds, Status Code: 400)"

Additional Information:

• The actual error message is visible when deploying the same template directly through CloudFormation
• This makes debugging deployment issues much more difficult as users need to check CloudFormation console or deploy separately to see the actual error
• Copilot version: v1.34.0
• AWS Region: eu-west-1
• Operating System: macOS

Workaround:
Currently users need to either:

• Check CloudFormation console directly for the actual error message
• Deploy the template directly using AWS CloudFormation to see the error

Suggested Fix:
Modify Copilot to properly capture and display the underlying CloudFormation error messages instead of showing "null".

[fuzzbuzzz]

Please find a screenshot from the CloudFormation events tab for reference. Thank you