Case for fallible cs() method

[skaunov]

Error handling is mocked just for example; return type for the modified method have been left incompatible as it shows the idea to the point.

Description

It's a small example on possible inconsistency when cs() is called on [T] which contains different non-null CS. I'd like to continue on this one if introduction of error handling is indeed reasonable in the crate; or receive feedback showing this is a bad idea.

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Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (master)
• Linked to Github issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Wrote unit tests
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

Unchecked boxes are due to the draft nature of this PR and it's purpose isn't merging but feedback to continue development in this direction.