Added a log message in core/output if the cache directory config value was not found.
diff --git a/system/core/Output.php b/system/core/Output.php
index e25e621..7d3e2e1 100644
--- a/system/core/Output.php
+++ b/system/core/Output.php
@@ -346,6 +346,7 @@
 		
 		if ( ! is_dir($cache_path) OR ! is_really_writable($cache_path))
 		{
+			log_message('error', "Unable to write cache file: ".$cache_path);
 			return;
 		}