Search pagination: decide better behavior for cloning/missing/timedout/partial repos

Created by: slimsag

After https://github.com/sourcegraph/sourcegraph/pull/4796 is merged, when the following condition is true:

len(batchCommon.cloning) > 0 || len(batchCommon.missing) > 0 || len(batchCommon.timedout) > 0 || len(batchCommon.partial) > 0

The result of the paginated request in this case is questionable for the user and we may want to consider returning an error or asking the user to retry the request later. This needs more thought.