datamotion: snapshot failure diagnostics unhidden (#3666)

Diagnostics are hard when a snapshot fails if a null pointer occurs. This is because no stack trace or location of the error is logged. I.E.
2019-10-21 12:55:00,056 DEBUG [o.a.c.s.m.AncientDataMotionStrategy] (Work-Job-Executor-131:ctx-80420156 job-10033827/job-10033828 ctx-4864e2f5) (logid:21454564) copy snasphot failed: java.lang.NullPointerException

Author: DaanHoogland

engine/storage/datamotion/src/main/java/org/apache/cloudstack/storage/motion/AncientDataMotionStrategy.java

@@ -564,7 +564,7 @@ protected Answer copySnapshot(DataObject srcData, DataObject destData) {
             }
             return answer;
         } catch (Exception e) {
-            s_logger.debug("copy snasphot failed: " + e.toString());
+            s_logger.debug("copy snasphot failed: ", e);
             if (cacheData != null) {
                 cacheMgr.deleteCacheObject(cacheData);
             }