Print what field gives a serializa/deserialize error

[vrnvu]

I was trying to write the PR myself but the code is quite confusing for me sorry.

The idea is that instead of a generic error:

Invalid Type: byte array (expected: a sequence)

A serializing library should give you the position, in this case we should know what field we are trying to parse:

Invalid Type for field "pieces": byte array (expected: a sequence)