Fix account closing flow

This is a fix for https://app.getsentry.com/gittip/gittip/group/23491408/

Users who have money in their Gittip balance are supposed to choose a disbursement option when closing their account. Unfortunately some people just scroll down to click on the button, and in that case we're currently raising a `BalanceIsNotZero` exception that results in a 500 response which ends up logged in Sentry. This PR implements showing an error message to the user instead.