Tolerate multiple calls to entity.manage()

- Instead of exception, just warn and ignore.
- Required because on rebind if a policy creates an entity while still
  rebinding other entities (while calling onManagementStarting on each),
  then when iterating over all to call onManagementStarted it will fail
  if one of those referenced entities is already fully managed.
Merge pull request #495 from aledsage/fix/tolerate-multiple-manage-calls

Tolerate multiple calls to entity.manage()