Remember and surface compile errors when turning on

Summary: When turning on Fast Refresh, we might have a compile error in previously saved files. We used to ignore it until a file is saved again, leading to a confusing experience. This changes it so that we remember the last compile error, and show it _either_ when we get it (if Fast Refresh is off), or when we _turn it on_.

Reviewed By: cpojer

Differential Revision: D16359160

fbshipit-source-id: 8dc237563c2a271a23019a32ff85b57de99cfabe