signature reader does not return error when it fails

[jagobagascon]

Hi! I've found that the SignatureReader never fails when calling the Method function. It is returning nil when it should return the error. There are a couple of cases like this:

win32metadata/types/signature.go

Line 238 in 8566457

return MethodSignature{}, nil

win32metadata/types/signature.go

Line 247 in 8566457

return MethodSignature{}, nil

The method I'm reading is the Invoke method from the TypedEventHandler delegate: https://docs.microsoft.com/en-us/uwp/api/windows.foundation.typedeventhandler-2?view=winrt-22621

[tdakkota]

I've fixed it in #41. Feel free to re-open if the problem still exists.