Remove specific 404 messages in wrench-openapi.json

[henricasanova]

When we first designed the wrench-openapi.json for the WRENCH API, we entered specific error messages for the 404 response (like "workflow not found", "task not found"). But in fact, these errors are returned as part of a valid JSON response. We should just set all human-readable error messages for 404 to something like: "REST API endpoint not found (404)", or something like this. ALSO, some entries don[t even have a "404" spec. Perhaps it's safe to remove them all? After all, 404/405 have known meaning.... what's the point is saying "route not found" or "invalid"?