Improve error for docker exec & LXC

ping @crosbymichael 
Well (coming from web-development), feature detection is always preferred over "browser sniffing". Basically, software using a driver should not know (or *assume*) that a driver does or *doesn't* support a feature, but *detect* if that feature is supported.

I realise that (at this stage) it's probably just nitpicking, but it will clutter the code in future if more checks are added. Also, those check can become invalid if, at a certain point a driver *does* support a feature (but the check still *assumes* it doesn't)

Anyway, I'm *not* a contributor, just a enthusiastic docker user, so I'm fine with any decision made, just wanted to echo my thoughts on this.

Keep up the fantastic work you're doing!