Print log when cannot poll from ListeningURLFile

I got this [flaky test](https://teamcity.cockroachdb.com/buildConfiguration/Cockroach_BazelEssentialCi/9027231?showRootCauses=true&expandBuildProblemsSection=true&expandBuildTestsSection=true) (internal link for ci) with `panic: empty connection string` error. It's hard to know why this happened as the server should be in `stateRunning`. If we can print out more log from the server that would be helpful.