Nuke use of `error` in the library code

[relrod]

We shouldn't use error but instead should return an Either or similar and correctly handle the case where we don't get a value back in the calling code.

ricky@localhost 99% ~/dev/haskell/habsim/haskell (master)$ git grep error | grep Data | grep -vE ':\s*--'
src/Data/HABSim/Internal.hs:  | alt <= (-500) = error "Altitude out of range, too low (<500m)"
src/Data/HABSim/Internal.hs:  | otherwise     = error "Altitude out of range, too high (>86km)"