Fix the CLI output of formal verification runs

[mdqst]

I noticed that the icons for success and error states were reversed. Currently, :x: is displayed for success (success == 0) and :heavy_check_mark: for errors (success != 0). This is counterintuitive, as :x: typically represents failure and :heavy_check_mark: success.

I've corrected the logic to align with common conventions.

Now, :heavy_check_mark: is shown for success and :x: for errors.

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Latest commit: 0cd2dad

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