Make error pages opt-in

[daffl]

As seen in feathersjs-ecosystem/feathers-mongoose#56 it is very easy to make a request and get an error page which looks like a mistake. Services on the other hand always return JSON. I'm wondering if we should make the HTML error pages opt-in (passing the filename in the options) and add them to the generator instead.

[marshallswain]

I would definitely appreciate this.

[ekryski]

It may sound stupid but I think it’s actually sort of important to support this by default because it gives people a sense of security and makes Feathers look polished. Like, ​_”man these guys have thought of everything. Even minimal error pages”_​.

If you don't want to use the middleware you can easily just write your own or use a different express error handler.

That being said I think we should do 3 things:

• you should be able to turn off HTML with something like html:false
• we should also check to see if the request was XHR and if it doesn't accept JSON assume the user forgot to set the Accepts header.
• We should provide a default.html page that show a "General Error" for any non 404 or 500 errors.

[marshallswain]

That sounds reasonable.

[daffl]

Opting in has been added in #25