rename coro Error.h to Result.h
rename coro Error.h to Result.h

Summary: [folly] Rename coro `Error.h` to `Result.h` since it has multiple result types.

Reviewed By: Mizuchi

Differential Revision: D25625178

fbshipit-source-id: 45f8194922b764429969329563a4695b1c13da6f

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift