Error handling - 404 when id or api endpoint is not found

[rlskoeser]

On my initial tests, API requests returned 500 errors when no data was loaded; it should have error handling throughout.

• magazines/id should return a 404 Not Found when the referenced id is not found
• issues/id should return a 404 Not Found when the referenced id is not found

Any other API endpoints that reference ids should return 404s if the id is not found in the loaded data.

Any requested URL that doesn't match a valid API endpoint should return a 404.

[rlskoeser]

In some cases, I got empty non-error pages (200 OK) for what I think were invalid urls - that makes it harder to figure out what's going wrong when experimenting with the API. These kinds of urls should either return a 404 or redirect somewhere useful.

[rlskoeser]

Notes from my last round of testing after updates were made:

• I'm getting a 204 No Content response for an invalid (nonexistent) constituent id, should be 404 Not Found (while it may be technically true that no content is returned, 20x is a successful status and it would be much more helpful for a consumer to get an indication that there was an error)
• I'm getting a 400 Bad Request response for an invalid/nonexistent issue or id, should be 404 Not Found (looks like it is coded to return 400 in the xquery module - now that I look, I see this in several places)