Error Page

The error page in the template, is just an information about the owner?
Because I already customized an error page...

Also, how can I change the default text in the page?

Did you try to check the error page in template?

/templates/ja_donate/error.php

There are some language text that you can translate:

JERROR_LAYOUT_PLEASE_TRY_ONE_OF_THE_FOLLOWING_PAGES
JERROR_LAYOUT_GO_TO_THE_HOME_PAGE
JERROR_LAYOUT_HOME_PAGE