Ignore NamespaceNotFound errors when running collMod during checkReplicaSet in replsettest.js

When running checkReplicaSet in replsettest.js, we run collMod on all the collections to wait for any index builds before running awaitReplication. However, if there is a background thread running a command, it could potentially drop a collection after listCollections but before collMod, causing a "NamespaceNotFound" error.

To fix this, check the output of runCommand to see if there was a "NamespaceNotFound" error and if so, ignore it.