Continue discovery on connect errors (fixes #324)

Continues trying to connect to the discovery server at regular intervals despite failure at initialization time (#324).

Whether or not to retry and the retry interval itself should be specified in the configuration, but this feature is not implemented in this commit.

Additionally, I have not managed to test this change in an isolated way because the version I based it on results in an `element size exceeded` error when reading from all of my remotes. I _can_, however, confirm that it does allow `syncthing` to establish a connection to the global discovery server even when it is not available at initialization time (e.g., when run `@reboot` in `crontab`).