Fails if privileges are limited

In a production environment, users usually have limited privileges. In this case kubectl tree <kind> <name> fails with

Error: error while querying api objects: listing resources failed (<GVK>): <kind> is forbidden

I would suggest to print a warning for 403s but continue processing otherwise.

EDIT: Just saw there is already #55 for this. Please merge.

[ahmetb]

Just pushed some changes to #55. Do you have the means to try that branch out and see if it works as you expected?