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If the python file raises an exception we print it like Python would.

```
$ ./target/debug/dynamo-run in=http out=pystr:~/Temp/cn47/1_e.py --model-name test

Traceback (most recent call last):
  File "/home/graham/Temp/cn47/1_e.py", line 17, in generate
    raise MyException("The message")
1_e.MyException: The message
```