Improves telemetry bean exception handling

[scottopell]

Each Instance creates and manages a Telemetry bean. This PR improves the error messages when the telemetry bean cannot be registered for whatever reason.

[rayz]

jmxfetch/src/main/java/org/datadog/jmxfetch/Instance.java

Lines 550 to 557 in 69cfd18

	instanceTelemetryBean.setBeansFetched(beans.size());
	instanceTelemetryBean.setTopLevelAttributeCount(matchingAttributes.size());
	instanceTelemetryBean.setMetricCount(metrics.size());
	log.debug("Updated jmx bean for instance: " + this.getCheckName()
	+ " With beans fetched = " + instanceTelemetryBean.getBeansFetched()
	+ " top attributes = " + instanceTelemetryBean.getTopLevelAttributeCount()
	+ " metrics = " + instanceTelemetryBean.getMetricCount());
	return metrics;

would it make sense to have a null check here?

[scottopell]

re #488 (comment)

In the current code structure it is impossible for the telemetry bean to be null. Even if there is an error while registering, the bean itself is still returned and set on the instance.
If you'd like to submit a PR with a null check I'd approve it, but its not necessary IMO