track dependencies from files with errors separately
from when the file doesn't have an error

More precisely, keep two sets of depndencies for a given
open tab. If the file expanded without error then we consider
that authoratative and keep that set of dependencies around
even if a later expansion fails. So there are two sets of
dependencies for each tab. When expansion succeeds, we reset
both of them, but if expansion fails then we reset only one,
preserving the set we got from the last successful expansion

please include in the release