To cache IOException when isTryCache

Currently, if some IOException happens when writing cache to Tachyon, it will block the whole writing operation. No matter the cache type is TRY or MUST.  Here, we just try to catch those IOE when WRITETYPE == TRY_CACHE, so that even the worker is broken, all those data can be checkpointed onto the underFS successfully. 

Besides, we also delete a member variable named "mCanCache". Since this variable is always true after it is initialized.  It doesn't play any role in the current implementation.