helper/schema: skip ValidateFunc on other errors

Guarantees that the `interface{}` arg to ValidateFunc is the proper
type, allowing implementations to be simpler.

Finish the docstring on `ValidateFunc` to call this out.

/cc @mitchellh

helper/schema/schema.go

@@ -132,7 +132,10 @@ type Schema struct {
 	// what do to about the removed attribute.
 	Removed string
 
-	// ValidateFunc allows individual fields to validate
+	// ValidateFunc allows individual fields to define arbitrary validation
+	// logic. It is yielded the provided config value as an interface{} that is
+	// guaranteed to be of the proper Schema type, and it can yield warnings or
+	// errors based on inspection of that value.
 	ValidateFunc SchemaValidateFunc
 }
 
@@ -1183,7 +1186,7 @@ func (m schemaMap) validateType(
 			"%q: [REMOVED] %s", k, schema.Removed))
 	}
 
-	if schema.ValidateFunc != nil {
+	if len(es) == 0 && schema.ValidateFunc != nil {
 		ws2, es2 := schema.ValidateFunc(raw)
 		for _, w := range ws2 {
 			ws = append(ws, fmt.Sprintf("%q: %s", k, w))

helper/schema/schema_test.go

@@ -3438,6 +3438,23 @@ func TestSchemaMap_Validate(t *testing.T) {
 				fmt.Errorf(`"validate_me": something is not right here`),
 			},
 		},
+
+		"ValidateFunc not called when type does not match": {
+			Schema: map[string]*Schema{
+				"number": &Schema{
+					Type:     TypeInt,
+					Required: true,
+					ValidateFunc: func(v interface{}) (ws []string, es []error) {
+						t.Fatalf("Should not have gotten validate call")
+						return
+					},
+				},
+			},
+			Config: map[string]interface{}{
+				"number": "NaN",
+			},
+			Err: true,
+		},
 	}
 
 	for tn, tc := range cases {