Refactor: Deduplicate error classes in log

CityDoctorParent/CityDoctorValidation/src/main/java/de/hft/stuttgart/citydoctor2/check/Checker.java

@@ -782,7 +782,7 @@ public class Checker {
 
     private List<GmlId> getMissedFeatures(List<Future<GmlId>> futures) throws InterruptedException{
         List<GmlId> missedList = new ArrayList<>();
-        Set<ExecutionException> errors = new HashSet<>();
+        Set<String> errors = new HashSet<>();
         for (Future<GmlId> future : futures) {
             try{
                 GmlId gmlId = future.get();
@@ -792,7 +792,7 @@ public class Checker {
             } catch (ExecutionException e){
                 logger.debug("A Task failed due to an unexpected exception", e);
                 logger.debug(e.getCause());
-                errors.add(e);
+                errors.add(e.getCause().toString());
             }
         }
         if (!errors.isEmpty()) {