Check for exceptions also when @returns() is used, not only for "->" return type annotations. (GH-3664)

When you use Python type annotations, it would be weird if you lost Python exception propagation semantics along the way, just by compiling the code. So the default behaviour is "except? -1" here for C integer types.

Arguably, this would also be a better default for the decorator case.

See https://github.com/cython/cython/issues/3625#issuecomment-631931675