Unregistered query non-helpful log spam

Running Consul 1.7.3

On some hosts I've seen a bit of spam in the Consul logs that looks like this:

```
2020-05-13T07:07:29.670Z [ERROR] agent.client: RPC failed to server: method=PreparedQuery.Execute server=x.x.x.x:8300 error="rpc error making call: Query not found"
2020-05-13T07:07:29.672Z [ERROR] agent.client: RPC failed to server: method=PreparedQuery.Execute server=x.x.x.x:8300 error="rpc error making call: Query not found"
```

I've been able to reproduce the error in the log by running something like this:

```
dig foo.query.consul. @127.0.0.1 -p 8600
```

I can see the logs being handy for debugging (the hosts in question run Nomad jobs, and I can't figure out which job is causing it), but as stands they don't contain any helpful information like the name of the query to troubleshoot the issue. The error level should probably be dropped to be lower than ERROR too.

Seems similar to #1660, but I'd posit that the logs are potentially useful, just not in their current form (or at their current log level).