Continue discovery on connect errors (fixes #324)

Continues trying to connect to the discovery server at regular intervals despite failure.

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`).