checker: don't cast match branch structs to IError when in sumtype

[edam]

Fixes #22435.

When checking match branch expression branch expression type, there was a check to cast yielded value to an IError when match return type is a Result and branch expression type is struct.

I added an additional test so that the cast (and check to ensure the struct conforms to the IError interface) does not occur when the return type is a sumtype and the struct is one of the sumtype types.

Tests updated to cover this

_{Huly®: V_0.6-20905}