[Resolves #1388] Handle failures gracefully in stack outputs

[alex-harvey-z3q]

This adds exception handling in the case of failures to parse the given arguments to !stack_output and !stack_output_external.

PR Checklist

• Wrote a good commit message & description [see guide below].
• Commit message starts with [Resolve #issue-number].
• Added/Updated unit tests.
• Added/Updated integration tests (if applicable).
• All unit tests (poetry run tox) are passing.
• Used the same coding conventions as the rest of the project.
• The new code passes pre-commit validations (poetry run pre-commit run --all-files).
• The PR relates to only one subject with a clear title.
  and description in grammatically correct, complete sentences.

Approver/Reviewer Checklist

• Before merge squash related commits.

Other Information

Guide to writing a good commit

[iainelder]

How does it look when you run it with a real template? Does it print a stack trace, or something more readable?

[iainelder]

If you provide a minimal Sceptre project setup that invokes the error, I'll run it at the command line to see how it feels compared to the original behavior.

[jfalkenstein]

Looks good now. Thanks for your work on this!

[zaro0508]

/integration-tests