VpcNetwork should throw a descriptive error when selecting 0 subnets

[MichaelHindley]

Trying to set up a ecs cluster with asg in a existing non-cdk managed vpc.
The externalVpc is used without issue when working with Lambdas, but with ASG it fails due to AvailabilityZones being empty, but they do exist and they do work with at least one other resource.

const externalVpc = ec2.VpcNetwork.import(this, 'exampleVPC', {
  availabilityZones: [cdk.Fn.importValue('az1'), cdk.Fn.importValue('az1')],
  publicSubnetIds: [cdk.Fn.importValue('publicsubnet1'), cdk.Fn.importValue('publicsubnet2')],
  vpcId: cdk.Fn.importValue('vpcId'),
});

// Cluster config
const cluster = new ecs.Cluster(this, 'cluster-test', {
  clusterName: 'cluster-test',
  vpc: externalVpc,
});

cluster.addCapacity('asg-test', {
  instanceType: new ec2.InstanceType('t3.small'),
  taskDrainTimeSeconds: 0,
});

results in

 CREATE_FAILED        | AWS::AutoScaling::AutoScalingGroup

 Property AvailabilityZones cannot be empty.
        new AutoScalingGroup (code\aws\node_modules\@aws-cdk\aws-autoscaling\lib\auto-scaling-group.ts:297:29)
        \_ Cluster.addCapacity (code\aws\node_modules\@aws-cdk\aws-ecs\lib\cluster.ts:78:30)
        \_ new ContainerRegistryStack (ode\operator-interface\aws\lib\container-repository-stack\ecr.ts:32:13)
        \_ Object.<anonymous> (code\operator-interface\aws\bin\aws.ts:12:1)
        \_ Module._compile (internal/modules/cjs/loader.js:689:30)
        \_ Object.Module._extensions..js (internal/modules/cjs/loader.js:700:10)
        \_ Module.load (internal/modules/cjs/loader.js:599:32)
        \_ tryModuleLoad (internal/modules/cjs/loader.js:538:12)
        \_ Function.Module._load (internal/modules/cjs/loader.js:530:3)
        \_ Function.Module.runMain (internal/modules/cjs/loader.js:742:12)
        \_ startup (internal/bootstrap/node.js:283:19)
        \_ bootstrapNodeJSCore (internal/bootstrap/node.js:743:3)

The actual error is that vpcPlacement: { subnetsToUse: SubnetType.Public } is missing in the config to config.addCapacity.

vpcPlacement is an optional parameter, but without it you get an error in this case.

[rix0rrr]

Yes, VpcNetwork should throw a descriptive error if it cannot find any subnets of the indicated type.

[rix0rrr]

Note to future implementors: not in all cases; sometimes consumers will do analysis on the list of subnets returned, and it's okay for it to be empty.

Honestly, it's probably better to add an API subnetIds(vpcPlacement): string[] which has the default validating behavior. This will satisfy the 9 out of 10 cases where constructs just want to take a list of subnets and stick them into a CFN resource, leaving constructs that have fallbacks to use the other function.