Better error message for conflicting settings in plando

[fenhl]

Since #1562, this plando:

{
    "settings": {
        "bridge": "vanilla",
        "bridge_medallions": 6
    }
}

produces this error message:

ValueError: 'bridge_medallions' must be set to 0 since 'bridge' is set to 'vanilla'

This is misleading because 0 is not a valid option for bridge_medallions, and the actual fix is to remove bridge_medallions from the plando. This PR updates the error message accordingly:

ValueError: The 'bridge_medallions' setting cannot be used since 'bridge' is set to 'vanilla'