Consider adding error handling for ParseComputeOperationInfo

[scotthart]

The call sites for ParseComputeOperationInfo do not provide the ability to propagate an error if ParseComputeOperationInfo detected one, such as a malformed URL. Consider enhancing the async get and async cancel request setting mechanism in AsyncRestLongRunningOperation to handle errors and refactor ParseComputeOperationInfo to return a StatusOr<T>.