Adds proper error message if birthday was given in the wrong format

What does this MR do?

We get a lot of support mails of people specifying their birthday in the wrong format. Currently, they get the following error message:

Aus datenschutz- und haftungsrechtlichen Gründen musst du mindestens 18 Jahre alt sein, um bei foodsharing.de mitzumachen.

This MR adds a clearer error message which should help people to solve the issue on their own.

How confident are you it won't break things if deployed?

Very, just adds an additional error message.

Links to related issues

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))
• Once your MR has been merged, you are responsible to update the #foodsharing-beta Slack channel about what has been changed here. They will test your work in different browsers, roles or other settings