Updates: Fail fast if HTTP proxy is not running (ticket #1530)

...erconsole/java/src/net/i2p/router/update/NewsFetcher.java

...e/java/src/net/i2p/router/update/PluginUpdateChecker.java

...le/java/src/net/i2p/router/update/PluginUpdateRunner.java

...java/src/net/i2p/router/update/UnsignedUpdateChecker.java

.../java/src/net/i2p/router/update/UnsignedUpdateRunner.java

...rconsole/java/src/net/i2p/router/update/UpdateRunner.java

...ole/java/src/net/i2p/router/web/ConfigClientsHandler.java

...sole/java/src/net/i2p/router/web/ConfigUpdateHandler.java

...terconsole/java/src/net/i2p/router/web/PluginStarter.java