Add error identification in deserializer code

This PR introduces some simple error identification by setting
appropriate variables.

Targets #467 partly, as it removes **some** `@todo`s from
`src/serialize/json/deserializer_callbacks.c`.
I think, this should probably two static variables and a pointer to one of them.
I'm not quite sure whether the compiler will build the executable by using one string location and always pointing to the same offset or whether the string will be created multiple times.