rather than Either and structured error type [Dependency]. The reason
we cannot use that as a structured error type any more is because
missing dependencies is not the only failure reason. There are
several reasons, several of which are pretty complex. For now we'll
have to do with a human readable message. Perhaps we may be able to
find a common structured type that the different dep resolvers can
all agree on. I'm not hopeful however as error reporting seems to be
closely tied to the dep resolution approach.