Add retry loop for Ensembl 'ping' check

Ensembl's REST service provides a 'ping' URL to check availability, https://rest.ensembl.org/info/ping?content-type=application/json.
Normally this should respond fast, but sometimes it is slow on the first request and fast on a second. Jalview should retry a couple of times, instead of reporting 'Ensembl is not available' on the first failure.