yamux error perhaps should be warn, not error

We are using consul as a HA backend for Vault and we keep getting this error every ever few days:

`[ERROR] agent: yamux: Failed to write header: tls: use of closed connection
`

I don't understand why this is happening at all and I can't find any documentation online about it. As far as I can tell everything works as normal and it doesn't seem something to be worried about when we get these errors so perhaps this should be of WARN type, not ERROR?

There was [an issue ](https://github.com/hashicorp/consul/issues/3040)open of this kind earlier that addressed this, but perhaps it can be taken a little further.