Scope down rescue on BoxCollection.find

After https://github.com/hashicorp/vagrant-plugin-sdk/pull/175 is merged
it will properly return a NotFound so we can rescue that specifically
and let any unexpected errors propagate.

plugins/commands/serve/client/box_collection.rb

@@ -52,7 +52,7 @@ module VagrantPlugins
             res = client.find(SDK::BoxCollection::FindRequest.new(
               name: name, version: version, providers: Array(providers)
             ))
-          rescue
+          rescue GRPC::NotFound
             logger.debug("box not found!")
             return nil
           end