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