users that are in the admins group and do not exist in the users group should be skipped over

[haidangwa]

In our chef server, we had one org that had a user listed in their admins group. However, further examination revealed that this admin user did not exist in the users group. This condition caused knife-tidy to abort with this error:

Fetching admins users for organisation ORGNAME
  Cannot fetch admin users for organisation ORGNAME as it does not exist on the server
ERROR: knife encountered an unexpected error
This may be a bug in the 'tidy notify' knife command or plugin
Please collect the output of this command with the `-VV` option before filing a bug report.
Exception: NoMethodError: undefined method `map' for nil:NilClass

This GET resulted in a 404 error: DEBUG: Initiating GET to https://chef.domain.com/organizations/ORGNAME/users/USERNAME