Include map key in error message

[wanewang]

I remember that 2.0 has key information in error message right?
like if let id:String = map["id"], but map["id"] is nil,
the message will have the key id in it.
It would be better for debugging with the key information.

[loganwright]

@jhihguan I thought I was able to preserve that, I'll take a look.