Expose firecracker errors more clearly

Right now, if `firecracker` has an error, we show a pretty cryptic message like `ctr: Put http://localhost/machine-config: dial unix /tmp/firecracker.sock: connect: no such file or directory: unknown`. I've been using the following wrapper script around the `firecracker` binary to save its output and get a better handle at diagnosing what went wrong, but we need to do a better job of making this accessible.

workaround wrapper script

```
$ cat /usr/local/bin/firecracker-wrapper
#!/bin/sh

exec > /tmp/firecracker.log
exec 2>&1

echo "Options: $@"

exec /usr/local/bin/firecracker "$@"
```