fix(toolkit): only fail if errors are on selected stacks

[rix0rrr]

If we report synthesis errors, they should only prevent the
affected stack from deploying; right now, since they are checked
during the synthesis step, they would abort the entire toolkit
run regardless of whether they would be selected or not.

Makes it possible to do region-dependent verification.

Fixes #1784.

ALSO IN THIS COMMIT

If the 'cdk synth' output goes to the screen, don't automatically
select upstream stacks for synthesis (as that would lead to an
immediate error).

Fixes #1783.

---

Pull Request Checklist

• Testing
  • Unit test added (prefer not to modify an existing test, otherwise, it's probably a breaking change)
  • CLI change?: coordinate update of integration tests with team
  • cdk-init template change?: coordinated update of integration tests with team
• Docs
  • jsdocs: All public APIs documented
  • README: README and/or documentation topic updated
• Title and Description
  • Change type: title prefixed with fix, feat will appear in changelog
  • Title: use lower-case and doesn't end with a period
  • Breaking?: last paragraph: "BREAKING CHANGE: <describe what changed + link for details>"
  • Issues: Indicate issues fixed via: "Fixes #xxx" or "Closes #xxx"
• Sensitive Modules (requires 2 PR approvers)
  • IAM Policy Document (in @aws-cdk/aws-iam)
  • EC2 Security Groups and ACLs (in @aws-cdk/aws-ec2)
  • Grant APIs (only if not based on official documentation with a reference)

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license.