Return 404 if project page table doesn't exist

[cpeel]

Instead of return a 500 server error, return a 404 if an API call is querying a project's page table that doesn't exist.

# current code returns 500
curl -i -H "Accept: application/json" -H "X-API-KEY: review" \
    -X GET "https://www.pgdp.org/api/v1/projects/projectID4b8bc04f5dc7c/pagedetails"

# new code returns 404
curl -i -H "Accept: application/json" -H "X-API-KEY: review" \
    -X GET "https://www.pgdp.org/~cpeel/c.branch/return-404-for-missing-project-table-api/api/v1/projects/projectID4b8bc04f5dc7c/pagedetails"

This fixes the following API endpoints that use the page table (using an archived project as an example):

/api/v1/projects/projectID4b8bc04f5dc7c/pagedetails
/api/v1/projects/projectID4b8bc04f5dc7c/pages
/api/v1/projects/projectID4b8bc04f5dc7c/pages/001.png/pagerounds/P1

Sandbox: https://www.pgdp.org/~cpeel/c.branch/return-404-for-missing-project-table-api/