Example in the readme doesn't show how to create erorr instances

[kornelski]

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The example shows InvalidToolchainName being defined, but this enum isn't used anywhere else in the code. It's unclear how instance of InvalidToolchainName error is actually created.

quick_error implements From so types are automagically converted, but in Failure's example both enum cases contain just a plain String, so I don't think ? can create any of these errors. Therefore, I think example is missing the most important bit of the code.

[Fedcomp]

What is funny - documentation says a lot about many things but not about such basics.