selectSubnet throws when trying to select a type that wasn't created [aws-ec2]

[pszabop]

When attempting to call ec2.vpc.selectSubnet(), which from the name sounds like a query, if no subnets exist given the criteria an error is thrown instead of an empty set returned.

Reproduction Steps

const fileSystemVpc = new ec2.Vpc(this, 'vpcForEfs', {
  natGateways: 0,
  maxAzs: 2,
});

// THIS ASSERTS and is a bug in ec2.vpc
// it should just return an empty set or undefined
const privateSubnets = fileSystemVpc.selectSubnets({
  subnetType: ec2.SubnetType.PRIVATE,
});

What did you expect to happen?

I expected an empty set or undefined returned

What actually happened?

Error There are no 'Private' subnet groups in this VPC. Available types: Isolated,Public was thrown

Environment

• **CLI Version :1.63
• **Framework Version:1.63
• **Node.js Version:12.16.1
• **OS :Linux
• **Language (Version):Typescript 3.7.5

Other

I have branches in my code that would like to know whether certain subnets exist or not, and right now I have to catch/throw for this exception as a work around. The exception is hard-coded in the aws-ec2 module and doesn't follow exception
best practices so I have to use a regular expression to look for it rather than looking up an exception type in a map or object. This makes the code fragile since you could change the text at any time.

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This is 🐛 Bug Report

[rix0rrr]

Take into account for #5927

[github-actions]

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[ramesh82]

I am currently using try catch to handle the exception but it would be good to return empty or null if the subnet types/groups doesn't exist.