Handle FailedPrecondition errors from the control plane (#948)

The control plane may return a FailedPrecondition error when the cluster
is in an unexpected state -- when there are more than one pod for a
single IP address. The proxy currently retries these lookups ad nauseum.

This change updates the proxy's error handling to avoid retries when
this error is returned, as if the IP were a non-cluster resource.

Author: olix0r

linkerd/app/core/src/lib.rs

@@ -70,7 +70,10 @@ pub fn discovery_rejected() -> tonic::Status {
 
 pub fn is_discovery_rejected(err: &(dyn std::error::Error + 'static)) -> bool {
     if let Some(status) = err.downcast_ref::<tonic::Status>() {
+        // Address is not resolveable
         status.code() == tonic::Code::InvalidArgument
+            // Unexpected cluster state
+            || status.code() == tonic::Code::FailedPrecondition
     } else if let Some(err) = err.source() {
         is_discovery_rejected(err)
     } else {