Precalculate the manager for unfetched dependencies
Revert "Fix status code in mix test"

This reverts commit 4fc3fc11de37dff41f439140e027a7743e00dc3e.

We want to run all mix umbrella projects together, because
all their apps are booted together and we should not pretend
otherwise.
Improve error message for overriden deps
Improve syntax error on invalid calls

Closes #1847