truncate JSTimers errors array instead of making it nullable
truncate JSTimers errors array instead of making it nullable

Summary:
instead of `errors: ?Array<Error>` and setting it back to `null` to clear errors, which Flow is not very happy with, we can make it always an array and truncate it with `errors.length = 0`.

Changelog: [Internal]

Reviewed By: motiz88

Differential Revision: D33584184

fbshipit-source-id: 81b424e69e60203c645bafbac12177ffcdc0c6ef

javache/react-native,facebook/react-native,facebook/react-native,janicduplessis/react-native,javache/react-native,janicduplessis/react-native,janicduplessis/react-native,facebook/react-native,janicduplessis/react-native,javache/react-native,javache/react-native,janicduplessis/react-native,facebook/react-native,facebook/react-native,facebook/react-native,javache/react-native,javache/react-native,janicduplessis/react-native,facebook/react-native,janicduplessis/react-native,facebook/react-native,javache/react-native,facebook/react-native,javache/react-native,janicduplessis/react-native,javache/react-native