libtock: Consistent error code for Invalid exit
libtock: Consistent error code for Invalid exit

When reviewing #170, I noticed that there's a mix of `1` and `-1`
used to express the same error (I think)?

Looks like this came as part of the big batch change in 77e90c1f.

tock/libtock-c,tock/libtock-c,tock/libtock-c