i2ptunnel: Improve message on filter file error

apps/i2ptunnel/java/src/net/i2p/i2ptunnel/I2PTunnelServer.java

@@ -233,7 +233,7 @@ public class I2PTunnelServer extends I2PTunnelTask implements Runnable {
             try {
                 _filter = FilterFactory.createFilter(context, filterDefinition);
             } catch (IOException | InvalidDefinitionException bad) {
-                throw new IllegalArgumentException("Can't create socket manager "+bad.getMessage(), bad);
+                throw new IllegalArgumentException("Bad filter definition file: " + bad.getMessage(), bad);
             }
         }