How to return an error message to the client via `input_fn`

[dshahrokhian]

What did you find confusing? Please describe.
In none of the examples I've read, I have found an example of returning an error to the client, when the requested format is not supported.

I have tried raising ValueError within the method, but then the client just receives error 500, not the reason.

Describe how documentation can be improved
Including in readthedocs (for example, this documentat) an example on how we can return a particular error message to the client.

[laurenyu]

@dshahrokhian sorry for the delay in response here. what framework and framework version are you using? (the answer varies based on that)