Convert-ShouldThrow: Fix ExpectedMessage

[johlju]

Pull Request (PR) description

This Pull Request (PR) fixes the following issues

Task list

• Added an entry to the change log under the Unreleased section of the
  file CHANGELOG.md. Entry should say what was changed and how that
  affects users (if applicable), and reference the issue being resolved
  (if applicable).
• Documentation added/updated in README.md and source/WikiSource.
• Comment-based help added/updated for all new/changed functions.
• Localization strings added/updated in all localization files as appropriate.
• Examples appropriately added/updated.
• Unit tests added/updated. See DSC Community Testing Guidelines.
• Integration tests added/updated (where applicable). See
  DSC Community Testing Guidelines.
• New/changed code adheres to DSC Community Style Guidelines.