Make postprocessors non-throwing

[SimplyDanny]

Failing immediately when a property is invalid is too strict. It feels sufficient to allow to report an issue but otherwise continue with a default value instead of stopping execution completely.

[SwiftLintBot]

	1 Warning
⚠️	If this is a user-facing change, please include a CHANGELOG entry to credit yourself! You can find it at CHANGELOG.md.

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Here's an example of your CHANGELOG entry:

* Make postprocessors non-throwing.  
  [SimplyDanny](https://github.com/SimplyDanny)
  [#issue_number](https://github.com/realm/SwiftLint/issues/issue_number)

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