In order to understand FailPoint's block open and close behavior,

I found it very helpful to rename several internal entities and reduce the RetCode enum.

The diff was too big for the ticket it was originally written for and was reverted in the interest of reviewer mercy, but here it is.

https://mongodbcr.appspot.com/748170001/#ps719300001

I think a standalone review just focusing on that patch would be very beneficial.

FailPoint has grown up a lot and some of these names don't cleanly apply.
For instance, all the ShouldFail stuff is really used for functions besides shouldFail, like executeIf and scopedIf.