Surface better error message when the stackset instance is not created successfully

When users do copilot env init and if the stackset instance was not created successfully when adding the env to the stackset, we won't know this until deploying the env stack when this error occurs

✘ get app resources: describing application resources: getting outputs for stack  in region eu-north-1: describe stack : ValidationError: 2 validation errors detected: Value '' at 'stackName' failed to satisfy constraint: Member must satisfy regular expression pattern: [a-zA-Z][-a-zA-Z0-9]*|arn:[-a-zA-Z0-9:/._+]*; Value '' at 'stackName' failed to satisfy constraint: Member must have length greater than or equal to 1
	status code: 400, request id: 3e07d746-06bc-4a7f-b781-6e8e3e07913d

which makes no sense to the users. Ideally after adding the env to the stackset, we need to do something to make sure the stackset instance is created successfully before we deploy the env stack.