Finally, the code now works for seeds coming from both form-data and a file.

Have now stopped the progress, as I cannot seem to fix the problem with the add-seeds-from-file functionality.
I have also created a review

This process is now in progress, and it works already for seeds read from a textarea

The easiest idea is probably to handle the request handling from HarvestDefinition/Definitions-edit-selective-harvest.jsp instead of from HarvestDefinition/Definitions-add-event-seeds.jsp and then write the invalid seeds to the screen as we done, when adding domains to a selective-harvest:

The following domains are illegal and cannot be added
sss

And html:

<br/>
<table class="selection_table">
    <tr>
        <th>
            The following domains are illegal and cannot be added
        </th>
    </tr>
    <tr>
        <td>
            <textarea rows="1" cols="30">sss</textarea>
        </td>
    </tr>
</table>