Update errorMessage option documentation with falsy value behaviour

[m-green]

Updates the description of the errorMessage option (in all the component pages it appears) so that it's clearer that a falsy value will be ignored.

Please check if factually correct.

[NickColley]

Epic nice work

[hannalaakso]

Looks great! Should we have an example of how to do this eg.

{% if error %}
  {% set errorMessage = { text: "error!" } %}
{% endif %}
...
{{ govukDateInput({
  errorMessage: errorMessage,
}) }}

I'm not sure if it's immediately clear how you should do this without an example.

What do you think @edwardhorsford?

[edwardhorsford]

@m-green thanks for adding.

@hannalaakso I'm tempted to think the description added is the most important part - as this is more relevant to developers implementing.

If you ever get to doing full worked pages, it might be helpful though.

[hannalaakso]

🎉