code: func EqualError(t TestingT, theError error, errString string, msgAndArgs ...interface{}) {
  	if h, ok := t.(tHelper); ok {
  		h.Helper()
  	}
  	if assert.EqualError(t, theError, errString, msgAndArgs...) {
  		return
  	}
  	t.FailNow()
  }
docs: // EqualError asserts that a function returned an error (i.e. not `nil`)
  // and that it is equal to the provided error.
  //
  //   actualObj, err := SomeFunction()
  //   assert.EqualError(t, err,  expectedErrorString)