Friendly error messages

[depaolim]

If some settings are not specified, error messages should be more explicative

Examples:

• settings.MEDIA_SERVERS

[depaolim]

If settings.MEDIA_SERVERS is not defined, default could be...

MEDIA_SERVERS = {
    'default': {
        'ENGINE': "auth_media.backends.interim",
    },
}

which is usefull for development environments