Detect internet connection loss for Map Tile server

[DSheirer]

Detect when the internet connection is lost and show a dialog to the user, versus spamming the log with unknown host errors.

20180216 152205.109 [map tile fetcher-pool-6] ERROR o.j.s.m.AbstractTileFactory - Failed to load a tile at url: http://tile.openstreetmap.org/12/1180/1503.png, retrying [45MB/72MB 62%]
java.net.UnknownHostException: tile.openstreetmap.org
at java.net.AbstractPlainSocketImpl.connect(Unknown Source)
at java.net.PlainSocketImpl.connect(Unknown Source)
at java.net.SocksSocketImpl.connect(Unknown Source)
at java.net.Socket.connect(Unknown Source)
at java.net.Socket.connect(Unknown Source)
at sun.net.NetworkClient.doConnect(Unknown Source)
at sun.net.www.http.HttpClient.openServer(Unknown Source)
at sun.net.www.http.HttpClient.openServer(Unknown Source)
at sun.net.www.http.HttpClient.(Unknown Source)
at sun.net.www.http.HttpClient.New(Unknown Source)
at sun.net.www.http.HttpClient.New(Unknown Source)
at sun.net.www.protocol.http.HttpURLConnection.getNewHttpClient(Unknown Source)
at sun.net.www.protocol.http.HttpURLConnection.plainConnect0(Unknown Source)
at sun.net.www.protocol.http.HttpURLConnection.plainConnect(Unknown Source)
at sun.net.www.protocol.http.HttpURLConnection.connect(Unknown Source)
at sun.net.www.protocol.http.HttpURLConnection.getInputStream0(Unknown Source)
at sun.net.www.protocol.http.HttpURLConnection.getInputStream(Unknown Source)
at java.net.URL.openStream(Unknown Source)
at org.jdesktop.swingx.mapviewer.AbstractTileFactory$TileRunner.cacheInputStream(AbstractTileFactory.java:391)
at org.jdesktop.swingx.mapviewer.AbstractTileFactory$TileRunner.run(AbstractTileFactory.java:341)
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
at java.util.concurrent.FutureTask.run(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)