“Goodbye Christopher Robin” is an exquisite, beautiful film, and like most beautiful things, there’s something painful about it. It depicts a kind of beauty, innocence and purity that can’t be forever, whose existence forces you to stop and appreciate it now — and in the moment of appreciating it, to contemplate its future nonexistence. That’s really the governing emotion of this film, the pitch that it reaches and sustains from beginning to end, a kind of sadness in the midst of happiness, a paradise with an awareness of mortality.