Detect expectError statements in nested blocks

[BendingBender]

Currently, expectError detection only checks statements in the outermost layer of a file. This misses all expectError statements that are nested inside of blocks (like if or try) producing false positives during linting.

This PR extends this detection mechanism to walk through the whole syntax tree to find expectError statements.

[SamVerschueren]

Cool!

Can you just explain in what use cases you would wrap it in something like an if-statement? I think I saw a TypeScript definition passing by with if-statements, but can't recall it anymore.

[BendingBender]

Here's one use case. Another would be for a discriminated union type. For such a type you need an if condition to differentiate between the shapes. Here's the use case for a discriminated union.

[SamVerschueren]

Thanks 🎉!