IPNS GetPublished should fail with unexpected error

#### Version information: 0.4.18

#### Type: Bug

#### Description:

When a new record is published, we try to get the its last published version, in order to update the sequence number. This way, taking into account the [GetPublished](https://github.com/ipfs/go-ipfs/blob/master/namesys/publisher.go#L110-L140) funtion, if the record is not found in the local datastore, we will use the routing to find it. If we do not find the record, all good, we will create a new one. However, if we receive any other error the publish should fail. Otherwise, we may restart the sequence number of a previously published record.

cc @alanshaw @Stebalien