api/v2/task/:taskId/review/start returns 500 if a review row isn't found

example request

GET | https://staging.maproulette.org/api/v2/task/170921569/review/start?mapillary=false

returns an ambiguous 500 error

message | "SqlMappingError(No rows when expecting a single one)"

This could be refined into something easier to understand for API users. I happened to run into this while going through a flow that isn't typically used by maproulette UI users (it's more of a modified dev flow for testing review pages). Regardless, here are the steps to reproduce...

Have 2 users, each logged in a different browser

Your User 1

1. start a task
2. mark 'Not an Issue'
3. In the confirmation modal, DO NOT check 'Request Review'
4. copy the URL
5. Press submit

Your User 2
6. paste the URL in a user 2's separate browser, adding /review to the end of it. submit the URL by pressing Enter
7. you should see the error, presented in the UI as 'Someone else is reviewing this challenge', which is false information (possible followup UI ticket).

Start by reproducing GET /api/v2/task/:taskId/review/start with the two-user flow described in the issue, then trace the endpoint handling for a missing review row. Confirm the response no longer exposes the ambiguous SqlMappingError and clearly communicates the missing-review condition to API users.