Improve error message when accessing packets during initialization

[ghost]

Per Robin:

I ran into something today that I thought y’all might be interested in. I was working on some derived code, and the C&T server kept crashing during the config load with a less-than-enlightening “no _dump_data is defined for class Thread::Mutex” error. Turns out this was caused by inadvertently reading a packet value during Conversion#initialize, which renders the packet un-dumpable since it creates a Mutex. While it’s probably best practice to avoid doing this (e.g. trying to read an undefined packet), it still seems like unexpected behavior that could cause headaches for others too.