* Remove dryStartCallback from sampleManager.
 * Unexport ErrStateEngineStopped->errStateEngineStopped.
 * Simplify multiError string when there's only one error.
 * Fix mistakenly swapped logging of dry start/ensure errors.

 * Add StateEngine.dryStarted check on Ensure() in order to make
   DryStart() call actually required.
 * Call DryStart() on overlord and state engine tests
 * Remove noopStateBackend: not required since --dry run is cut
   after the DryStart() call, so no state is ever persisted.
   Furthermore, this allows for potential dry-run of state patches.
 * Remove Overlord.addManager(), since StateEngine.AddManager()
   already exists for this purpose, and Overlord.AddManager() as well.
 * Don't plan.ReadDir() on ServiceManager.DryStart(), acquire plan
   instead so that we actually initialize the manager instead.