set_callbacks does not require all items to be callable

[robberwick]

Expected Behavior

set_callbacks() should fail gracefully if all members are not callable

Actual Behavior

set_callbacks() does not fail when all members are not callable, and the code attempts to invoke the specified item when its index in the callback list corresponds with the packet_id

Steps to Reproduce the Problem

1. call set_callbacks(), passing a list of non-callable items e.g. strings
2. set_callbacks invocation does not fail
3. send SerialTransfer packet with a packet_id whose index corresponds with an item in the list of callbacks
4. call tick()
5. script crashes, as callback item is not callable