Unfortunately this is currently not supported.
There is an accepted issue for this. It seems to be not trivial to implement (see quote from the ticket below) but hopefully it comes with Go 1.3.
Made slightly inconvenient to implement because by the time the error is generated the decoder is looking at the field value itself, not the struct that contains the field. The relevant code doesn't even know that it's decoding into a struct field. There are a few ways to get around this but they are non-trivial.