Move exception raising back into retryable block in VirtualBox driver command execution

For some reason the actual raising of the exception was moved outside of the retryable block by @mitchellh in commit bc2a9fe780e44fcf3fb989178b0582a8f2514895 which broke the ability to retry VirtualBox commands if they fail and are marked as retryable.

I discovered this issue when looking at issue #1622 and discovered that this solution for that issue, retrying the command, was not functioning.