Infer error state from message text

[joelvdavies]

e.g. In

  const [nameError, setNameError] = React.useState(false);
  const [nameErrorMessage, setNameErrorMessage] = React.useState<
    string | undefined
  >(undefined);

Remove

const [nameError, setNameError] = React.useState(false);

and instead infer the state using

nameErrorMessage !== undefined

See #80 (comment)