code: func EqualErrorf(t TestingT, theError error, errString string, msg string, args ...interface{}) bool {
  	if h, ok := t.(tHelper); ok {
  		h.Helper()
  	}
  	return EqualError(t, theError, errString, append([]interface{}{msg}, args...)...)
  }
docs: // EqualErrorf asserts that a function returned an error (i.e. not `nil`)
  // and that it is equal to the provided error.
  //
  //   actualObj, err := SomeFunction()
  //   assert.EqualErrorf(t, err,  expectedErrorString, "error message %s", "formatted")