finish Injector refactoring: replace all exceptions with Either

[Kai]

@pshirshov

[Paul S.]

I've removed all the unnecessary exceptions and internal usages of planUnsafe (former plan), the uses which are still there have to throw anyway, so it's ok to leave it as it is.

One use may be removed if we support bindings like fromEither:

  make[AppStartupPlans].from {
    (roleAppPlanner: RoleAppPlanner, roots: Set[DIKey] @Id("distage.roles.roots")) =>
      roleAppPlanner.makePlan(roots)
  }

makePlan may return Either here but that would be useless unless we have .fromEither.