Handle server response when downloading plugin properly

[xsalefter]

There's case when custom plugin URL return non-trivial HTTP errors: 4xx or 5xx.

My experience was, I tried to download plugin from URL, and server return with 400 response code with message: Download Restricted: DOWNLOADS_EXCEEDED The maximum number of downloads has been exceeded.

The log, on the other side, only give us generic error:

org.killbill.billing.osgi.bundles.kpm.KPMPluginException: org.killbill.billing.plugin.util.http.InvalidRequest: Invalid request
	at org.killbill.billing.osgi.bundles.kpm.impl.DefaultPluginManager.install(DefaultPluginManager.java:159)
	at org.killbill.billing.osgi.bundles.kpm.EventsListener.handleKillbillEvent(EventsListener.java:96)
	at org.killbill.billing.osgi.libs.killbill.OSGIKillbillEventDispatcher$1.handleKillbillEvent(OSGIKillbillEventDispatcher.java:98)
	at org.killbill.billing.osgi.libs.killbill.OSGIKillbillEventDispatcher$1.update(OSGIKillbillEventDispatcher.java:84)
	at org.killbill.billing.osgi.KillbillEventObservable.notifyObservers(KillbillEventObservable.java:39)
	at org.killbill.billing.osgi.KillbillEventObservable.setChangedAndNotifyObservers(KillbillEventObservable.java:45)
	at org.killbill.billing.osgi.KillbillEventRetriableBusHandler$1.run(KillbillEventRetriableBusHandler.java:82)
	at org.killbill.billing.osgi.KillbillEventRetriableBusHandler$1.run(KillbillEventRetriableBusHandler.java:77)
	at org.killbill.queue.retry.RetryableSubscriber$SubscriberQueueHandler.handleReadyNotification(RetryableSubscriber.java:89)
	at org.killbill.queue.retry.RetryableHandler.handleReadyNotification(RetryableHandler.java:49)
	at org.killbill.queue.retry.RetryableSubscriber.handleEvent(RetryableSubscriber.java:49)
	at org.killbill.billing.osgi.KillbillEventRetriableBusHandler.handleKillbillEvent(KillbillEventRetriableBusHandler.java:128)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:566)
	at org.killbill.commons.eventbus.Subscriber.invokeSubscriberMethod(Subscriber.java:106)
	at org.killbill.commons.eventbus.Subscriber.lambda$dispatchEvent$0(Subscriber.java:92)
	at org.killbill.commons.utils.concurrent.DirectExecutor.execute(DirectExecutor.java:34)
	at org.killbill.commons.eventbus.Subscriber.dispatchEvent(Subscriber.java:89)
	at org.killbill.commons.eventbus.Dispatcher$ImmediateDispatcher.dispatch(Dispatcher.java:132)
	at org.killbill.commons.eventbus.EventBus.postWithException(EventBus.java:293)
	at org.killbill.bus.DefaultPersistentBus.dispatchBusEventWithMetrics(DefaultPersistentBus.java:430)
	at org.killbill.bus.dispatching.BusCallableCallback.dispatch(BusCallableCallback.java:42)
	at org.killbill.bus.dispatching.BusCallableCallback.dispatch(BusCallableCallback.java:31)
	at org.killbill.queue.dispatching.Dispatcher$CallableQueueHandler.call(Dispatcher.java:146)
	at org.killbill.queue.dispatching.Dispatcher$CallableQueueHandler.call(Dispatcher.java:114)
	at org.killbill.commons.concurrent.WrappedCallable.call(WrappedCallable.java:45)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at org.killbill.commons.concurrent.WrappedRunnable.run(WrappedRunnable.java:49)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
	at java.base/java.lang.Thread.run(Thread.java:834)
Caused by: org.killbill.billing.plugin.util.http.InvalidRequest: Invalid request
	at org.killbill.billing.plugin.util.http.HttpClient.executeAndWait(HttpClient.java:198)
	at org.killbill.billing.plugin.util.http.HttpClient.doCall(HttpClient.java:183)
	at org.killbill.billing.osgi.bundles.kpm.KPMClient.download(KPMClient.java:63)
	at org.killbill.billing.osgi.bundles.kpm.KPMClient.download(KPMClient.java:59)
	at org.killbill.billing.osgi.bundles.kpm.impl.DefaultPluginManager.install(DefaultPluginManager.java:147)
	... 32 common frames omitted