rpc: /dial_peers: only mark peers as persistent if flag is on (#3620)

## Description

also

    handle errors from DialPeersAsync
    remove nil addr from log msg
    fix TestPEXReactorDoesNotDisconnectFromPersistentPeerInSeedMode

This is a follow-up from
#3593 (review)

Fixes most of the #3617, except #3593 (comment)

## Commits

* rpc: /dial_peers: only mark peers as persistent if flag is on

also

- handle errors from DialPeersAsync
- remove nil addr from log msg
- fix TestPEXReactorDoesNotDisconnectFromPersistentPeerInSeedMode

This is a follow-up from
https://github.com/tendermint/tendermint/pull/3593#pullrequestreview-233556909

* remove a call to AddPersistentPeers

TestDialFail will trigger a reconnect

node/node.go

@@ -682,8 +682,10 @@ func (n *Node) OnStart() error {
 	}
 
 	// Always connect to persistent peers
-	// parsing errors are handled above by AddPersistentPeers
-	_ = n.sw.DialPeersAsync(splitAndTrimEmpty(n.config.P2P.PersistentPeers, ",", " "))
+	err = n.sw.DialPeersAsync(splitAndTrimEmpty(n.config.P2P.PersistentPeers, ",", " "))
+	if err != nil {
+		return errors.Wrap(err, "could not dial peers from persistent_peers field")
+	}
 
 	return nil
 }