Add even more info

E.g.:

```
$ cat foo | http PATCH 127.0.0.1:2001/v2/images/PHPInfo/phpinfo-server
HTTP/1.1 400 Bad Request
...
{
    "errors": [
        {
            "message": "[jsonapi unmarshalNode]: Can't unmarshal map[min_containers:2 max_containers:5] () to struct field `Scaling`, which is a pointer to `Scaling` (struct), which is not a supported type",
            "status": 400
        }
    ]
}
```

Author: msabramo

request.go

@@ -33,8 +33,11 @@ var (
 
 // ErrUnsupportedPtrType is returned when the Struct field was a pointer but
 // the JSON value was of a different type
-func ErrUnsupportedPtrType(t interface{}) error {
-	return fmt.Errorf("Pointer (%s) in struct is not supported", t)
+func ErrUnsupportedPtrType(rf reflect.Value, f reflect.StructField) error {
+	return fmt.Errorf(
+		"[jsonapi unmarshalNode]: Can't unmarshal %+v (%s) to struct field `%s`, which is a pointer to `%s` (%s), which is not a supported type",
+		rf, rf.Type().Name(), f.Name, f.Type.Elem().Name(), f.Type.Elem().Kind(),
+	)
 }
 
 // UnmarshalPayload converts an io into a struct instance using jsonapi tags on
@@ -424,7 +427,6 @@ func unmarshalNode(data *Node, model reflect.Value, included *map[string]*Node)
 			// Field was a Pointer type
 			if fieldValue.Kind() == reflect.Ptr {
 				var concreteVal reflect.Value
-
 				switch cVal := val.(type) {
 				case string:
 					concreteVal = reflect.ValueOf(&cVal)
@@ -437,11 +439,11 @@ func unmarshalNode(data *Node, model reflect.Value, included *map[string]*Node)
 				case uintptr:
 					concreteVal = reflect.ValueOf(&cVal)
 				default:
-					return ErrUnsupportedPtrType(cVal)
+					return ErrUnsupportedPtrType(reflect.ValueOf(val), fieldType)
 				}
 
 				if fieldValue.Type() != concreteVal.Type() {
-					return ErrUnsupportedPtrType(fieldValue.Type())
+					return ErrUnsupportedPtrType(reflect.ValueOf(val), fieldType)
 				}
 
 				fieldValue.Set(concreteVal)

request_test.go

@@ -126,7 +126,7 @@ func TestUnmarshalToStructWithPointerAttr_BadType(t *testing.T) {
 	in := map[string]interface{}{
 		"name": true, // This is the wrong type.
 	}
-	expectedErrorMessage := ErrUnsupportedPtrType("*string").Error()
+	expectedErrorMessage := "[jsonapi unmarshalNode]: Can't unmarshal true (bool) to struct field `Name`, which is a pointer to `string` (string), which is not a supported type"
 
 	err := UnmarshalPayload(sampleWithPointerPayload(in), out)