Automatic merge from submit-queue (batch tested with PRs 38805, 42362, 42862)

Let GC print specific message for RESTMapping failure

Make the error messages reported in https://github.com/kubernetes/kubernetes/issues/39816 to be more specific, also only print the message once.

I'll also update the garbage collector's doc to clearly state we don't support tpr yet.

We'll wait for the watchable discovery feature (@sttts are you going to work on that?) to land in 1.7, and then enable the garbage collector to handle TPR.

cc @hongchaodeng @MikaelCluseau @djMax