code: func (a *Assertions) EqualErrorf(theError error, errString string, msg string, args ...interface{}) {
  	if h, ok := a.t.(tHelper); ok {
  		h.Helper()
  	}
  	EqualErrorf(a.t, theError, errString, 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()
  //   a.EqualErrorf(err,  expectedErrorString, "error message %s", "formatted")