Improve error for docker exec & LXC

ping @crosbymichael 
Sorry if this is a silly comment (I'm not a GO-programmer), but wouldn't it make more sense to make the *driver* responsible for indicating if this is supported? e.g. `if !d.execDriver.supportsExec() { .....`  or `if !d.execDriver.supports('exec') { .... ` 

Seems more future-proof, and exec.go doesn't have to keep track of *which* driver supports *what*.