Return error from Group.Wait

Changes the return value of Group.Wait to error instead of *Error. The *Error concrete type can lead to unintuitive, subtle bugs around nil checks (see: https://golang.org/doc/faq#nil_error). Returning the error interface instead eliminates these. Addresses hashicorp#57.

group.go

@@ -6,7 +6,7 @@ import "sync"
 // coalesced.
 type Group struct {
 	mutex sync.Mutex
-	err   *Error
+	err   error
 	wg    sync.WaitGroup
 }
 
@@ -30,7 +30,7 @@ func (g *Group) Go(f func() error) {
 
 // Wait blocks until all function calls from the Go method have returned, then
 // returns the multierror.
-func (g *Group) Wait() *Error {
+func (g *Group) Wait() error {
 	g.wg.Wait()
 	g.mutex.Lock()
 	defer g.mutex.Unlock()

group_test.go

@@ -42,3 +42,38 @@ func TestGroup(t *testing.T) {
 		}
 	}
 }
+
+func TestGroupWait_ErrorNil(t *testing.T) {
+	g := new(Group)
+	g.Go(func() error { return nil })
+	err := g.Wait()
+	if err != nil {
+		t.Fatalf("expected error to be nil, but was %v", err)
+	}
+}
+
+func TestGroupWait_ErrorNotNil(t *testing.T) {
+	g := new(Group)
+	msg := "test error"
+	g.Go(func() error { return errors.New(msg) })
+	err := g.Wait()
+	if err == nil {
+		t.Fatalf("expected error to be nil, but was %v", err)
+	}
+
+	// err is a *Error, and e is set to the error's value
+	var e *Error
+	if !errors.As(err, &e) {
+		t.Fatalf("expected err to be type *Error, but was type %T, value %v", err, err)
+	}
+
+	errs := e.WrappedErrors()
+	if len(errs) != 1 {
+		t.Fatalf("expected one wrapped error, but found %d", len(errs))
+	}
+
+	wrapped := errs[0]
+	if wrapped.Error() != "test error" {
+		t.Fatalf("expected wrap error message to be '%s', but was '%s'", msg, wrapped.Error())
+	}
+}