Include error message with exception when the agent fails to start. (p…

…rometheus#399)

Signed-off-by: Jon Haddad <jon@jonhaddad.com>

jmx_prometheus_javaagent/src/main/java/io/prometheus/jmx/JavaAgent.java

@@ -31,7 +31,7 @@ public static void premain(String agentArgument, Instrumentation instrumentation
             server = new HTTPServer(config.socket, CollectorRegistry.defaultRegistry, true);
         }
         catch (IllegalArgumentException e) {
-            System.err.println("Usage: -javaagent:/path/to/JavaAgent.jar=[host:]<port>:<yaml configuration file>");
+            System.err.println("Usage: -javaagent:/path/to/JavaAgent.jar=[host:]<port>:<yaml configuration file> " + e.getMessage());
             System.exit(1);
         }
     }