Add initial error output

Adds a small message in case you login with the wrong credentials.

@lautenbach what are the recommended style elements here?

Additionally, I read a bit more into Django forms and it should be possible to adjust the template a bit to use the field information from Django: https://docs.djangoproject.com/en/4.1/ref/forms/api/#attributes-of-boundfield? (also see the other link in the comment of the MR)

Closes #2 (closed)

Closes SDC-748