Standardize error messages across the app

[0xCLARITY]

Similar to JSDoc comments #131 and code comments #132, we should do a pass through all the error messages we return in the app and just make sure we're being consistent in our communication, and following best practices for error messages.

Best practices for error messages in an API include a link to documentation for Bad Requests, and possibly an example good request so the client knows how to fix the problem.