input validation

[gadicc]

e.g. currently with invalid input you might get:

[2022-11-21 16:54:27 +0000] [7] [ERROR] Exception occurred while handling uri: 'http://0.0.0.0:8000/'
Traceback (most recent call last):
  File "handle_request", line 81, in handle_request
    FutureStatic,
  File "server.py", line 36, in inference
    output = user_src.inference(model_inputs)
  File "/app.py", line 173, in inference
    startRequestId = call_inputs.get("startRequestId", None)
AttributeError: 'NoneType' object has no attribute 'get'

not obvious what's going on there unless you know the code well, we should rather emphasize the expected format like we started doing in docker-diffusers-api.