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Click turns functions into "commands" which has the following consequences:
- Any return statement means that it has succeeded whatever the value returned
  therefore the exit code will be 0
- To "fail" properly an exception must be raised which will return an exit
  code of 1
- Other exceptions will be treated as usage error and have an exit code
  of 2

The raise_on_error decorator will take the returned value of the wrapped
function and raise a generic error with the exit code set to what the
function has returned.