Add NamespaceUnavailableFailure (#481)

**Why?**

To differentiate between a namespace that's temporary unavailable and not active.
This new failure will be returned with a gRPC unavailable error and should be retried by clients automatically.

Author: bergundy

temporal/api/errordetails/v1/message.proto

@@ -56,6 +56,13 @@ message NamespaceNotActiveFailure {
     string active_cluster = 3;
 }
 
+// NamespaceUnavailableFailure is returned by the service when a request addresses a namespace that is unavailable. For
+// example, when a namespace is in the process of failing over between clusters.
+// This is a transient error that should be automatically retried by clients.
+message NamespaceUnavailableFailure {
+    string namespace = 1;
+}
+
 message NamespaceInvalidStateFailure {
     string namespace = 1;
     // Current state of the requested namespace.
@@ -130,4 +137,4 @@ message MultiOperationExecutionFailure {
         string message = 2;
         repeated google.protobuf.Any details = 3;
     }
-}
+}