[Polling tests] Add Handle unexpected errors

[anxolin]

Normally, validations, if done right, will catch all errors and return a comprehensive error.

This is why, by the time we do the call to getTradeableOrderWithSignature we will have quite some certainty the call will succeed. If it doesn't we will consider it an UNEXPECTED_ERROR and will return the original error with the result.

This PR adds a unit test to validate we get this result in this situation.

[github-actions]

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[mfw78]

Minor nit on in-line logic of comment.