provide a better missing error message

When github say's missing, it's not talking about the resource - it means something referenced in the data.

For example, specifying a milestone that doesn't exist will return:

```
{
	"message": "Validation Failed"
	"documentation_url": "http"://developer.github.com/v3"
	"errors": [
		{
			"value": 42,
			"resource": "Issue",
			"field": "milestone",
			"code": "missing"
		}
	]
}
```

Previously this would be reported as:

> Validation Failed: Resource "Issue" not exists anymore

Now it's reported as

> Validation Failed: The milestone 42 does not exist, for resource "Issue"