OO-291 fix missing error on wrong group id

src/main/java/org/olat/group/ui/main/BusinessGroupSearchController.java

@@ -216,7 +216,7 @@ public class BusinessGroupSearchController extends FormBasicController implement
 				try {
 					new Long(id.getValue());
 				} catch (NumberFormatException e) {
-					id.setErrorKey("", null);
+					id.setErrorKey("search.id.format", null);
 					allOk &= false;
 				}
 			}

src/main/java/org/olat/group/ui/main/_i18n/LocalStrings_de.properties

@@ -51,6 +51,7 @@ search.no=Nein
 search.resources=In Kursen verwendet
 search.headless=Orphans
 search.headless.check=Gruppe ohne Mitglieder und Ressourcen
+search.id.format=Keine g\u00FCltige Gruppen-ID
 cif.displayname=Name
 cif.description=Beschreibung
 cif.owner=Betreuer