Handle errors in mail configuration

[etj]

The e-mail verification button may display a generic error if the mail server settings are wrong or set as default values
e.g.

DJANGO_EMAIL_HOST=localhost
DJANGO_EMAIL_HOST_PASSWORD=
DJANGO_EMAIL_HOST_USER=
DJANGO_EMAIL_PORT=25

The error is something like this (disregard the custom CSS):

We may want to display a proper error if the EMAIL settings are wrong or the connection to the mail server is failing.

[giohappy]

@etj this is a message for the user, not admins, so I wouldn't share technical details about the error.
We might define a list of error codes that will be included in this message and that users can share with admins, in case.