Inspiron 15 (3521) -- hard drive "failed" on Support Assist self-test

Hi everyone, 

So today the Support Assist self-test for my Dell Inspiron 15 (3521) said that the hard drive had "failed" its test.  It gave the following error codes for the hard drive (ST500LT012-9WS142):

     WHD16-6GP  (SMART short self test:  "failed")

     WHD20-J2C  (Targeted read test:  "failed")

I've never had anything like this happen before.  So I'm just wondering:  Does anyone know what exactly this means?  And how serious is it?

Though running a little slower, it's still working fine for the moment.  But I don't want it to fail completely....  Whatever advice you all might have, I would greatly appreciate it.  Thanks!  - Robert

P.S. This laptop is considerably past the expiration of its warranty.