Os: Use Expected<T, SystemError> instead of return codes

[kachsheev]

Need:

• Implement SystemError for Windows/Linux
• Replace error codes to Expected<T, SystemError>

SystemError class:

class SystemError
{
public:
  SystemError(Status status) noexcept;

  Status code() const noexcept;
  const char *info() const noexcept;

private:
  const char *errorInfo;
  Status errorStatus;
};