Contextualize redefinition errors

[pelletier]

Follow up to #667. When SeenTracker finds a duplicate in the document structure it returns a simple error with no context. For example:

toml: key b is already defined

We should also provide more context about where the error occurred in the document, maybe the current path to the key at fault, possibly the source of the error (like where was the original key that just got redefined), similar to DecodeError (maybe should just reuse it?).