I would love to edit the text of the warning too.
Ability to translate system messages to other languages
Form => E-Mail Subject and E-Mail Text
This is the browser default error message with localization. You might need to build your own requirement error message to have this one fixed by yourself.
@lucaspchara that's the default browser validation text. The benefit of this is that it changes to fit your local language.
Here is some info on how to change this: