Handle yaml-cpp exceptions from a cloud-init config better

Currently, we just bubble up any `yaml-cpp` exceptions back to the client, but the error message is out of context of what is really wrong. For example, if a user creates a cloud-init config with incorrect yaml, an error could look like:
`launch failed: operator[] call on a scalar`
As can be seen, it's not clear where the error is.

We should catch the `yaml-cpp` exceptions and make it a more meaningful message to pass to the client and user.