Return XML parser error in metarefresh

To aid administrators in finding the root cause of an XML parse fail,
we should return the error message we got from the underlying parser.

modules/metarefresh/lib/MetaLoader.php

@@ -120,6 +120,7 @@ class sspmod_metarefresh_MetaLoader
 			$entities = $this->loadXML($data, $source);
 		} catch(Exception $e) {
 			SimpleSAML\Logger::debug('XML parser error when parsing ' . $source['src'] . ' - attempting to re-use cached metadata');
+			SimpleSAML\Logger::debug('XML parser returned: ' . $e->getMessage());
 			$this->addCachedMetadata($source);
 			return;
 		}