Disable next-button for unknown accounts

[phavekes]

The "Next" button is not usefull if no email exists for the enterd account.
Disable the button when the error is displayed, and re-enable it after editing the email address.

Change the text to
EN: "We could not find an eduID with that email. Check for typos or create a new eduID account."
NL: "We konden geen eduID vinden met deze mail. Check op typefouten, of maak een nieuw eduID aan."

[edwinvandeb]

Tekst ook aanpassen naar: "We could not find an eduID with that email. Check for typos or create a new eduID account."