LdapEndpoint value is not a string

#### Describe the bug
When in a LdapEndpoint an attribut is not a string the only message the user gets is "2019-06-14T09:26:58+00:00 WARNING ldap_modify_batch(): Each element of a 'values' array must be a string in /usr/share/tubee/vendor/dreamscapes/ldap-core/Dreamscapes/Ldap/Core/Ldap.php:533"
The workflow does not run correctly, so an additional warning/message would be good