[Resolve #1436] Handle exception in differ

[alex-harvey-z3q]

See also Issue #1336. Before this change, in the event of Parameters being entered of types other than str (e.g. bool, presumably float, int etc), the differ would bomb out with a confusing AttributeError error when trying to rstrip newline characters.

This changes to instead raise a SceptreException at that point, with an informative error message to help the caller understand what they did wrong.

PR Checklist

• [x] Wrote a good commit message & description [see guide below].
• [x] Commit message starts with [Resolve #issue-number].
• [x] Added/Updated unit tests.
• [ ] Added/Updated integration tests (if applicable).
• [x] All unit tests (poetry run tox) are passing.
• [x] Used the same coding conventions as the rest of the project.
• [x] The new code passes pre-commit validations (poetry run pre-commit run --all-files).
• [x] 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

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