Inconsistent behavior of openContext for Jsfs, nodefs and domfs

[rpaquay]

There is a difference in behavior between jsfs and (domfs, nodefs) wrt to calling openContextwhen path points to a non existent file:

• jsfs returns a rejected promise, where as (domfs, nodefs) return a fullfilled one
• (domfs, nodefs) return a rejected promise when calling open

We probably should pick one, and I would vote for the (domfs, nodefs) one as it makes createOpenContext a lighter weight operation.

[rpaquay]

Also,

• jsfs returns AxiomError.NotFound
• (domfs, nodefs) return AxiomError.Invalid (invalid path)