Show type for ErrUnsupportedPtrType

This may aid in figuring out where the problematic JSON is

Before:

```
Pointer type in struct is not supported
```

After:

```
Pointer (*int) in struct is not supported
```

request.go

@@ -27,13 +27,16 @@ var (
 	// (numeric) but the Struct field was a non numeric type (i.e. not int, uint,
 	// float, etc)
 	ErrUnknownFieldNumberType = errors.New("The struct field was not of a known number type")
-	// ErrUnsupportedPtrType is returned when the Struct field was a pointer but
-	// the JSON value was of a different type
-	ErrUnsupportedPtrType = errors.New("Pointer type in struct is not supported")
 	// ErrInvalidType is returned when the given type is incompatible with the expected type.
 	ErrInvalidType = errors.New("Invalid type provided") // I wish we used punctuation.
 )
 
+// 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)
+}
+
 // UnmarshalPayload converts an io into a struct instance using jsonapi tags on
 // struct fields. This method supports single request payloads only, at the
 // moment. Bulk creates and updates are not supported yet.
@@ -434,11 +437,11 @@ func unmarshalNode(data *Node, model reflect.Value, included *map[string]*Node)
 				case uintptr:
 					concreteVal = reflect.ValueOf(&cVal)
 				default:
-					return ErrUnsupportedPtrType
+					return ErrUnsupportedPtrType(cVal)
 				}
 
 				if fieldValue.Type() != concreteVal.Type() {
-					return ErrUnsupportedPtrType
+					return ErrUnsupportedPtrType(fieldValue.Type())
 				}
 
 				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.Error()
+	expectedErrorMessage := ErrUnsupportedPtrType("*string").Error()
 
 	err := UnmarshalPayload(sampleWithPointerPayload(in), out)