Fixed encoding issue in ErrorController

src/main/java/it/inaf/oats/vospace/exception/ErrorController.java

 package it.inaf.oats.vospace.exception;
 
+import java.nio.charset.StandardCharsets;
 import java.util.Map;
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
@@ -23,7 +24,11 @@ public class ErrorController extends AbstractErrorController {
     public void errorText(HttpServletRequest request, HttpServletResponse response) throws Exception {
         Map<String, Object> errors = super.getErrorAttributes(request, true);
         response.setContentType("text/plain;charset=UTF-8");
-        response.getOutputStream().print((String) errors.get("message"));
+        response.setCharacterEncoding("UTF-8");
+        String errorMessage = (String) errors.get("message");
+        if (errorMessage != null) {
+            response.getOutputStream().write(errorMessage.getBytes(StandardCharsets.UTF_8));
+        }
     }
 
     @Override