Do not throw error when get/read cache failed by Memcached::Rails

Memcached::Rails#get or read will throw timeout and other errors, which will cause 503 error in rails application; Rails' cache store will catch and ignore Timeout error from MemCache, however, there are errors wrapped as RuntimeError in Memcached.java and RuntimeException in spymemcached, so I simply do same thing with the Rails#set method, just ignore any error in get method.