Alert patron on lending error

[jimchamp]

Closes #8372

Displays a toast message when clicking a "Borrow" or "Read" button results in a 404 or 409 from the server. Otherwise, redirects to the location returned by the server. A json handler for ocaid checkouts has been added in support of this feature.

Technical

The borrow logic was moved into its own method, which, instead of raising exceptions, instead returns a two-tuple containing the HTTP status code and optional redirect location. This method is called by both the original ocaid checkout handler, and the new json handler.

Testing

Screenshot

Stakeholders

[jimchamp]

An improved version of this function would be useful throughout the codebase.

[mekarpeles]

I think there's a lot here that's in the right direction, my main reservation is having the borrow process itself require js -- my preference would be that we attempt a borrow and if it fails, we return the patron to the book page with a GET parameter like error=limit which then displays a toast message on page load. I think this would be much simpler code-wise, less brittle in the event of js errors, and not require js

[jimchamp]

@mekarpeles you'll probably want to update this copy. I think that you suggested something specific last week, but I was unable to find the suggestion.

[mekarpeles]

@bfalling your input required

[jimchamp]

There is no need for an error query parameter:

[mekarpeles]

LGTM, simple!