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?
Reading this again, I'm guessing this is the problem you were hitting in the PR description, you'd have likely been in dev mode and never hitting the handler.  If so, then I think the prod/dev split is reasonable, as long as Foreman renders the 500 properly (using the application exception handler) when in prod.