Add error handling to store plugin about annotator

[aron]

I've added a friendlier notifications to the Annotator class (perhaps it should reside in a separate object?) and added some error messages for various response statuses.

However it's looking like the XMLHTTPRequest object is not updated in either WebKit or Firefox if the request is not successful. This results in a status code of 0 (which jQuery normalises to 302).

Example

var xhr = new XMLHttpRequest();

xhr.open('GET', 'http://localhost:5000/annotations/21', true);
xhr.onreadystatechange = function () {
  console.log("%o %o", xhr.status, xhr.responseText, xhr)
};
xhr.send();

So unless anyone has any thoughts? It looks like we're only able to provide very generic error messages, which is a shame.

[aron]

Another option that may be worth considering is to detect the Annotator (using a custom HTTP header) and then always return a successful response but override the status in a similar way to method overloading in web frameworks.

For example an invalid authorisation would return

HTTP/1.1 200 OK
X-HTTP-Status-Override: 401 Unauthorized

Although this may not be worth the effort involved in updating the store.

[aron]

Closing this ticket. CORS error handling may need to be ticketed for improvement at a later date.

Current state is that Annotation.showNotification() displays a message fixed to the top of the users screen with the current error message. The notification can be removed on click or will hide itself after five seconds.

Generic error messages are:

"Sorry we could not #{action} the annotations from the store"

if the request affects more than one annotation and

"Sorry we could not #{action} this annotation"

where it affects just one. #{action} will be replaced with the current action eg. read, update, delete etc.

When we get a definite status from the server:

401 Unauthorised: "Sorry you are not allowed to #{action} this annotation"
404 Not Found: "Sorry we could not connect to the annotations store"
500 Internal Server Error: "Sorry something went wrong with the annotation store"

In future it would be useful to return more specific errors from the annotation store.