Add support to return value in StableMIR interface and not crash due to compilation error
Invoking `StableMir::run()` on a crate that has any compilation error was crashing the entire process. Instead, return a `CompilerError` so the user knows compilation did not succeed. I believe ICE will also be converted to `CompilerError`.
I'm also adding a possibility for the callback to return a value. I think it will be handy for users (at least it was for my current task of implementing a tool to validate stable-mir). However, if people disagree, I can remove that.
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