Indexing / Lower severity of getIndexField

While indexing, a number of log error can be reported:
```
2024-06-18T17:11:21,679 ERROR [geonetwork] - Failed to get index field 'resourceTitleObject' value on '10.17600/14000100', caused by Document with id 10.17600/14000100 not found
```
This is due to either remote record links not present in index eg. DOI or to an empty index and document search returns no document at starts.

It is triggered mainly in https://github.com/geonetwork/core-geonetwork/blob/main/web/src/main/webapp/xslt/common/index-utils.xsl#L141 but the `recordLink` field is currently not used in main UI.

Related to geonetwork@e606996 and geonetwork@20cc13b

core/src/main/java/org/fao/geonet/util/XslUtil.java

@@ -853,7 +853,7 @@ public static String getIndexField(Object appName, Object uuid, Object field, Ob
             final Map<String, String> values = searchManager.getFieldsValues(id, fields, language);
             return values.get(fieldname);
         } catch (Exception e) {
-            Log.error(Geonet.GEONETWORK, "Failed to get index field '" + fieldname + "' value on '" + id + "', caused by " + e.getMessage());
+            Log.warning(Geonet.GEONETWORK, "Failed to get index field '" + fieldname + "' value on '" + id + "', caused by " + e.getMessage());
         }
         return "";
     }