Rescue API handle NotImplementedError

There are several nova virt drivers that don't implement the rescue API,
but the os compute API doesn't handle NotImplementedError,
it returns a 400 instead of a 501.

The patch add the proper logic to return a 501 instead.

Change-Id: Ia649c4dadd50985efed631ce8f3e4b212646766e
Closes-Bug: #1287367
Merge "Rescue API handle NotImplementedError"