Notifying users about incorrect forward headers

[hershd23]

To the existing error message adding

1. What signature failed (input or output)
2. Providing a signature template to the user
3. Added link to the forward decorator in the EvaDB documentation

Pointing users to the following documentation
https://evadb.readthedocs.io/en/stable/source/reference/ai/custom-ai-function.html#part-1-writing-a-custom-function

[xzdandy]

Thanks for improving the error message!