Change an ascii error message to NOT_FOUND for ENGINE_KEY_ENOENT.
Change an ascii error message to NOT_FOUND for ENGINE_KEY_ENOENT.

This is required to avoid any client side changes.

Change-Id: I00259769a3521bdce2662192f2dce2f000adad6e

zbase/memcached,zbase/memcached,zbase/memcached,zbase/memcached,zbase/memcached