changed exit() to show_error() in _filter_uri()

http://codeigniter.com/bug_tracker/bug/6500/
diff --git a/system/libraries/URI.php b/system/libraries/URI.php
index aa2d71e..b95c74e 100644
--- a/system/libraries/URI.php
+++ b/system/libraries/URI.php
@@ -189,7 +189,7 @@
 			if ( ! preg_match("|^[".preg_quote($this->config->item('permitted_uri_chars'))."]+$|i", $str))
 			{
 				header('HTTP/1.1 400 Bad Request');
-				exit('The URI you submitted has disallowed characters.');
+				show_error('The URI you submitted has disallowed characters.');
 			}
 		}