fix(ec2): throw useful error when using lazy CIDR in VPC

[rix0rrr]

We can't use lazy (or deploy-time) CIDRs when creating a VPC, since we
need to subdivide the CIDR for all the subnets. Throw a descriptive
error message to inform users when they try to do this.

Fixes #3617.

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[mergify]

Pull Request Checklist

• Testing

• Unit test added (prefer to add a new test rather than modify existing tests)
• CLI change? Re-run/add CLI integration tests

• Documentation

• Inline docs: make sure all public APIs are documented (copy & paste from official AWS docs)
• README: update module README
• Design: for significant features, follow the design process

• Title uses the format type(scope): text

• Type: fix, feat, refactor go into CHANGELOG, chore is hidden
• Scope: name of the module without the aws- or cdk- prefix or postfix (e.g. s3 instead of aws-s3-deployment)
• Style: use all lower-case, do not end with a period

• Description

• Rationale: describe rationale of change and approach taken
• Issues: Indicate issues fixed via: fixes #xxx or closes #xxx
• Breaking?: last paragraph: BREAKING CHANGE: <describe what changed + link for details>

• Sensitive Modules (requires 2 PR approvers)

• IAM document library (in @aws-cdk/aws-iam)
• EC2 security groups and ACLs (in @aws-cdk/aws-ec2)
• Grant APIs (if not based on official documentation with a reference)