Separate input validation errors, FunctionInputHandlingException and other errors

[zootalures]

There are several cases where we throw a FunctionInputHandlingException on valid user input:

• The function format is invalid (not default or http), which is a configuration error
• We can't find an invoker that can run the function, which is a programming error
• We have an IOException reading the input stream, which is likely a programming error (read after close)

At the same time, we don't have an exception for a genuine input validation issue.

So we need to:

• separate programming/configuration issues from i/o issues, and use the right exceptions for those
• create a new exception to represent genuinely wrong input