error can be obscure when marshalling illegal values

[Roger Peppe]

Here's a distilled example of a situation I encountered for real
(I'd forgotten to call Interface() on the reflect.Value that
I was adding to the map).

package main
import (
 "fmt"
 "launchpad.net/goyaml"
 "reflect"
)
func main() {
 x := map[string]interface{}{"hello": reflect.ValueOf(56)}
 _, err := goyaml.Marshal(x)
 fmt.Printf("error: %v\n", err)
}

This prints:
error: YAML error: reflect.Value.Interface: cannot return value obtained from unexported field or method

I think it would be desirable to include some kind of context with the error
(for example the type that caused the problem).