Provide more information on deserialization Errors

[sebasmagri]

We're constantly getting errors from from_value like this one:

Error: internal error in native module: Failed to process input campaigns chunk: Err
or(Msg("invalid type: unit value, expected a string"), State { next_error: None, backtrace: None })

As we're processing and trying to deserialize quite big JSON objects, it's pretty hard to find which value got an unit value (null) instead of a String.

It'd be nice to have neon-serde providing more information of, at least, the key/property that failed to be deserialized.