ignore discovery errors

[bergerx]

Currently, it fails when any of the api-resources is failing but this should only be a warning, and there is no need to block the tree to proceed given we print some warnings about the error.

Current version:

$ kubectl  tree --version
kubectl version v0.4.3

$ kubectl tree StatefulSet/matst-gn7lu2-monitor
Error: failed to fetch api groups from kubernetes: unable to retrieve the complete list of server APIs: data.packaging.carvel.dev/v1alpha1: the server is currently unable to handle the request

With this PR:

$ ./kubectl-tree StatefulSet/matst-gn7lu2-monitor
E1208 00:07:06.589076 2603851 memcache.go:287] couldn't get resource list for data.packaging.carvel.dev/v1alpha1: the server is currently unable to handle the request
E1208 00:07:06.762539 2603851 memcache.go:121] couldn't get resource list for data.packaging.carvel.dev/v1alpha1: the server is currently unable to handle the request
NAMESPACE  NAME                                                  READY  REASON  AGE
default    StatefulSet/matst-gn7lu2-monitor                      -              59m
default    ├─ControllerRevision/matst-gn7lu2-monitor-55895d466c  -              59m
default    └─Pod/matst-gn7lu2-monitor-0                          True           59m

I just logged the error rather than printing it to the stderr as client-go already prints the warnings ootb as you can see above. But if you run it with -v 1, you can still see the relevant error:

$ ./kubectl-tree -v1 StatefulSet/matst-gn7lu2-monitor
E1208 00:06:56.769060 2603504 memcache.go:287] couldn't get resource list for data.packaging.carvel.dev/v1alpha1: the server is currently unable to handle the request
E1208 00:06:56.941860 2603504 memcache.go:121] couldn't get resource list for data.packaging.carvel.dev/v1alpha1: the server is currently unable to handle the request
I1208 00:06:56.942236 2603504 apis.go:49] failed to fetch api groups from kubernetes: unable to retrieve the complete list of server APIs: data.packaging.carvel.dev/v1alpha1: the server is currently unable to handle the request
NAMESPACE  NAME                                                  READY  REASON  AGE
default    StatefulSet/matst-gn7lu2-monitor                      -              59m
default    ├─ControllerRevision/matst-gn7lu2-monitor-55895d466c  -              59m
default    └─Pod/matst-gn7lu2-monitor-0                          True           59m

[ahmetb]

I hastily merged this but now I'm not sure if this was a good idea. :/

How do we know the API list (including builtin APIs) if client.ServerPreferredResources() fails to run? In your example, how does the code know about ControllerRevision or Pod APIs to iterate on?

Also I think any errors shouldn't be .V(1), they should be always visible IMO. @bergerx