Fix Unexpected error

[ksss]

If RBS::MixinClassError is raised during a steep check, the steep worker thread will be in a wait state forever.

With this fix, not only will steep not stop when RBS::MixinClassError is raised, but it will also show the problem.

mixin-class-error.rbs:5:2: [error] Cannot include a class `::Foo` in the definition of `::Bar`
│ Diagnostic ID: RBS::MixinClassError
│
└   include Foo
    ~~~~~~~~~~~

[soutaro]

Thank you! 🎉 🖊️

[soutaro]

Oops the implementation looks good, but still has test failure...

[soutaro]

@ksss The test failures should be fixed in #413. Could you try rebasing on the master branch?

[soutaro]

I run test locally and confirmed it passes.

[ksss]

Thanks ✨