Add an error message when user provides a non-existing file with sandbox definition as input

First-time users are puzzled by this Python error over and over:

Error: Cannot find a required file.
Traceback (most recent call last):
  File "C:\Users\misko\sandbox\sandbox-creator\create.py", line 16, in <module>
    generate_vagrantfile(device_definitions)
  File "C:\Users\misko\sandbox\sandbox-creator\modules\file_generator.py", line
60, in generate_vagrantfile
    device_definitions = create_devices(definitions)
  File "C:\Users\misko\sandbox\sandbox-creator\modules\device_creator.py", line
68, in create_devices
    **_create_hosts(definitions, mappings, flavors),
  File "C:\Users\misko\sandbox\sandbox-creator\modules\device_creator.py", line
37, in _create_hosts
    for host in yml['hosts']:
TypeError: 'NoneType' object is not subscriptable

Remove traceback.