engine: graph: Don't error when state file is missing

For some reason we get errors when we try to remove a non-existent state
file. There's a slight possibility that it could be a bug we're working
around, but it's not clear that this is the case, and I think it's
possible that a state file could have gotten nuked by the user somehow,
although this was occurring "naturally" when running reverse1.mcl so
let's keep that working for now.

engine/graph/reverse.go

@@ -291,5 +291,10 @@ func (obj *State) ReversalDelete() error {
 	}
 	file := path.Join(dir, ReverseFile) // return a unique file
 
-	return errwrap.Wrapf(os.Remove(file), "could not remove reverse state file")
+	// FIXME: why do we see these removals when there isn't a state file?
+	if err = os.Remove(file); os.IsNotExist(err) {
+		return nil // ignore missing files
+	}
+
+	return errwrap.Wrapf(err, "could not remove reverse state file")
 }