Fix EZP-23840: Incorrect error handling in elevate configuration

JIRA: https://jira.ez.no/browse/EZP-23840

Since SOLR 4.0+, error responses return an "error" structure (/array/object),
which can cause a fatal error when throwing a PHP exception with this non-string value.

This fixes it by using the appropriate error message key (`msg`) if applicable.

(see also: https://issues.apache.org/jira/browse/SOLR-141 )