BohemianCoding/Sketch#3319 Script import: Add support for absolute and ~ paths

Import paths can be in the format:

    relative/to/script.js
    /absolute/path/to/script.js
    ~/relative/to/home/script.js

Ref BohemianCoding/Sketch#3319
Not sure about this. Gus had a more specific error for the situation where base wasn't set.
I think we should preserve that error in that situation, but report the NS error otherwise.