:bug: Searching groups on a non existing application result in 500 error

When searching groups on a non existing application 500 is returned with error :

```
fr.insee.sugoi.model.exceptions.StoreException: search failed
at fr.insee.sugoi.store.ldap.LdapReaderStore.searchOnLdap(LdapReaderStore.java:345)
at fr.insee.sugoi.store.ldap.LdapReaderStore.searchGroups(LdapReaderStore.java:191)
at fr.insee.sugoi.core.service.impl.GroupServiceImpl.findByProperties(GroupServiceImpl.java:146)
at fr.insee.sugoi.services.controller.GroupController.getGroups(GroupController.java:116)
....
Caused by: com.unboundid.ldap.sdk.LDAPSearchException: Unable to perform the search because base entry 'ou=applitest2_Objets,ou=applitest2,ou=Applications,o=insee,c=fr' does not exist in the server.
at com.unboundid.ldap.sdk.LDAPConnection.search(LDAPConnection.java:3994)
at fr.insee.sugoi.store.ldap.LdapReaderStore.searchOnLdap(LdapReaderStore.java:330)

```
for the example of : /realms/domaine1/applications/applitest2/groups on the embedded test server
An existence check of the app should be done before requesting the ldap. Otherwise a check on groups might be done in the controller.