Missing ssh key in DO is not gracefully detected

[louis-reid]

It throws an exception:

Traceback (most recent call last):
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  File "/home/user/.shiv/machine_329452bc7e33e0c16f73d4584fbc07656468d6ab53b95adde6eb4696a2047b82/site-packages/machine/subcommands/create.py", line 54, in command
    user_data = get_user_data(manager, config.ssh_key, machine_config)
  File "/home/user/.shiv/machine_329452bc7e33e0c16f73d4584fbc07656468d6ab53b95adde6eb4696a2047b82/site-packages/machine/cloud_config.py", line 9, in get_user_data
    ssh_public_key = ssh_key.public_key
AttributeError: 'NoneType' object has no attribute 'public_key'