Found this while running `./vagrant box` with no args to get the help
output. It turns out you can have an empty RunResult but also a nil
error. I took the occasion to unwind the conditional tree a bit which
hopefully makes it a bit easier to read.
In legacy Vagrant, any exception raised that's a subclass of
Vagrant::Errors::VagrantError is considered user-facing and so causes
the error message to be printed to the console and the process to use
exit code 1. Anything outside of that causes the process to use exit
code 255. (See `bin/vagrant` for the code.)
Here we mirror that behavior by treating errors that have a
LocalizedMessage as user-facing and those without as unexpected. This
allows the basic virtualbox component to pass in vagrant-spec!
