TAMAYA-277: Fix warning

Use logger instead of dumping the stacktrace to System.err
diff --git a/code/core/src/main/java/org/apache/tamaya/core/internal/OSGIServiceContext.java b/code/core/src/main/java/org/apache/tamaya/core/internal/OSGIServiceContext.java
index 15c7b54..260bef1 100644
--- a/code/core/src/main/java/org/apache/tamaya/core/internal/OSGIServiceContext.java
+++ b/code/core/src/main/java/org/apache/tamaya/core/internal/OSGIServiceContext.java
@@ -130,7 +130,7 @@
                     this.osgiServiceLoader.getBundleContext().registerService(serviceType, t, new Hashtable<>());
                 }
             } catch (Exception e) {
-                e.printStackTrace();
+                LOG.log(Level.SERVERE, "Error while getting service.", e);
             }
         }
         return services;