Dinghy should not hide docker-machine errors

[fgrehm]

For example, when creating a VM using the xhyve provider and the xhyve driver is not set up, this is what we get on the console:

According to @zwily, that error did not show up when running dinghy create. There are likely other types of errors that can be surfaced to users but this seems like a place to start hunting them down 🔫

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Fork of #139 (comment)

[codekitchen]

Dinghy already prints out the stderr of the docker-machine create command if it fails, so I dug into this a bit more. Turns out it's a bug in the checking logic, so I filed machine-drivers/docker-machine-driver-xhyve#104

Closing this in favor of that fix. Thanks.

[fgrehm]

Awesome! Thanks for digging into it!