code: func Errorf(t TestingT, err error, msg string, args ...interface{}) bool {
  	if h, ok := t.(tHelper); ok {
  		h.Helper()
  	}
  	return Error(t, err, append([]interface{}{msg}, args...)...)
  }
docs: // Errorf asserts that a function returned an error (i.e. not `nil`).
  //
  //   actualObj, err := SomeFunction()
  //   if assert.Errorf(t, err, "error message %s", "formatted") {
  // 	   assert.Equal(t, expectedErrorf, err)
  //   }