Fixes #6099 - Bubble CR errors up the stack to the Host so we can display them

As it stands, this will cause "/compute_resources/:id/vms/new" to stop rendering CR errors nicely. We have two options:

1) Be happy with it not rendering nicely - it's an undocumented, unlinked page, for advanced users, and will at least still render the error, just not in a nice way
2) Add something like "orchestrated = true/false" to the create_vm() method, so we can tell if it was called directly or not, and then only rescue when orchestrated == false

I'm for 1) just to keep the code clean, at least the advanced users still get the error, even if it's unstyled.

@domcleal, @ohadlevy thoughts?
I have absolutely no idea. I assume it's for a good reason, and I don't want to introduce new bugs with this PR.