Improve error message upon Babushka error

When there's an error in the Babushka run, Vagrant prints out this error message:

> The following SSH command responded with a non-zero exit status.
> Vagrant assumes that this means the command failed!
> 
> %{command}
> 
> Stdout from the command:
> 
> %{stdout}
> 
> Stderr from the command:
> 
> %{stderr}

This is great as a general-purpose error message for when an SSH command exits with a non-zero exit status.

However, this is extremely confusing when the Babushka run has an error. Since Babushka loves to output to both stdout and stderr at any time, when Vagrant goes to show them separately it can be really confusing -- it's more valuable just to say "oops, there was an error, look above" since we've already been printing the stdout and stderr the whole time.