Properly report observable returning nullptr
Properly report observable returning nullptr

Summary: Make sure exception is propagated to caller (especially when Observer is just created).

Reviewed By: yfeldblum

Differential Revision: D3798991

fbshipit-source-id: cee8452cfd40dcfbaf0e2ae2c2ee628af0362c6e

Orvid/folly,facebook/folly,floxard/folly,charsyam/folly,rklabs/folly,facebook/folly,rklabs/folly,Orvid/folly,charsyam/folly,Orvid/folly,facebook/folly,rklabs/folly,Orvid/folly,floxard/folly,facebook/folly,charsyam/folly,floxard/folly,charsyam/folly,floxard/folly,rklabs/folly,facebook/folly,rklabs/folly,floxard/folly,Orvid/folly