[index.php] Filter parameter '_error_time' from queries

The parameter is used in error feeds. Since RSS-Bridge returns valid
feeds for error conditions, feed readers may attempt to access the
URI returned for the feed item in order to collect additional data,
thus including the parameter '_error_time' in the query.

This results in another error message, because it is an invalid input
parameter. Filtering the parameter allows RSS-Bridge to return the
original feed.

References #882

index.php

@@ -202,6 +202,7 @@ try {
 					'format',
 					'_noproxy',
 					'_cache_timeout',
+					'_error_time'
 				), '')
 		);
 
@@ -214,6 +215,7 @@ try {
 					'format',
 					'_noproxy',
 					'_cache_timeout',
+					'_error_time'
 				), '')
 		);