Refactor: Add logger message

CityDoctorParent/CityDoctorModel/src/main/java/de/hft/stuttgart/citydoctor2/zip/CityGmlZipArchive.java

@@ -50,7 +50,8 @@ public class CityGmlZipArchive implements Serializable {
             cgmlArchive.setEntries(archiveEntries);
             return cgmlArchive;
         } catch (IOException e) {
-            throw new RuntimeException(e);
+            logger.error("Encountered an IOError while registering CityGmlArchive", e);
+            return null;
         }
     }