Don't print a stacktrace when failing to get a metric

[albertvaka]

A multi-line stack trace was being printed to the log when a metric couldn't be retrieved, eg: because of a cast error. This makes the error fit into a single log line.

Before:

2019-04-11 17:59:46,641 | WARN | JmxAttribute | Unable to get metrics from org.datadog.jmxfetch.test:type=SimpleTestJavaApp - MyAwesomeMetric
java.lang.NumberFormatException: For input string: "ImNotANumber"
        at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:2043)
        at sun.misc.FloatingDecimal.parseDouble(FloatingDecimal.java:110)
        at java.lang.Double.parseDouble(Double.java:538)
        at org.datadog.jmxfetch.JmxAttribute.castToDouble(JmxAttribute.java:301)
        at org.datadog.jmxfetch.JmxSimpleAttribute.getMetrics(JmxSimpleAttribute.java:45)
        at org.datadog.jmxfetch.JmxAttribute.getMetricsCount(JmxAttribute.java:254)
        at org.datadog.jmxfetch.Instance.getMatchingAttributes(Instance.java:562)
        at org.datadog.jmxfetch.Instance.init(Instance.java:361)
        at org.datadog.jmxfetch.InstanceInitializingTask.call(InstanceInitializingTask.java:15)
        at org.datadog.jmxfetch.InstanceInitializingTask.call(InstanceInitializingTask.java:3)
        at java.util.concurrent.FutureTask.run(FutureTask.java:266)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
        at java.lang.Thread.run(Thread.java:748)

After:

2019-04-11 18:03:10,754 | WARN | JmxAttribute | Unable to get metrics from org.datadog.jmxfetch.test:type=SimpleTestJavaApp - MyAwesomeMetric: java.lang.NumberFormatException: For input string: "ImNotANumber"