Neither miner nor driver are shutting down the GPU it simply stops responding to commands. The GPU is almost like a self cointained computing environment (think OS for math). The host system gives the GPU a kernel to run, provides inputs, collects output, and periodically provides GPU control instructions. Other than that the GPU operates autonomously.
If the GPU is making errors some of those errors could manifest themselves in just computations (2+2=5) some can manifest themselves in flow control. The first will manifest themselves are HW error. The second can manifest themselves are an unresponsive GPU.
When a GPU crashes it often is still at full load. So it is doing "something" just not anything useful and no longer responding to any command and control signals from the driver. There is nothing you can do to avoid that other than not pushing GPU past their point of stability.
Aww. What a great post, completely spoiled by the ending.
Actually, would I even need cgminer to detect HW errors? I'm not familiar with it, but surely it just detects them the same way other miners do - when the GPU reports a good share, it sends it to the pool, who replies 'bzzt piss off this is crap', resulting in an invalid share? As it happens my pool reports very few stales (<0.5% long term) and NO duplicates or 'others' which is I presume what invalid shares are?
I dunno I'm kind of rambling here. What was my point again?