Make CMS errors JSON-able if requested via AJAX

cms/djangoapps/contentstore/views/error.py

-from django.http import HttpResponseServerError, HttpResponseNotFound
+from django.http import (HttpResponse, HttpResponseServerError,
+                         HttpResponseNotFound)
 from mitxmako.shortcuts import render_to_string, render_to_response
+import functools
+import json
 
 __all__ = ['not_found', 'server_error', 'render_404', 'render_500']
 
 
+def jsonable_error(status=500, message="The Studio servers encountered an error"):
+    """
+    A decorator to make an error view return an JSON-formatted message if
+    it was requested via AJAX.
+    """
+    def outer(func):
+        @functools.wraps(func)
+        def inner(request, *args, **kwargs):
+            if request.is_ajax():
+                content = json.dumps({"error": message})
+                return HttpResponse(content, content_type="application/json",
+                                    status=status)
+            else:
+                return func(request, *args, **kwargs)
+        return inner
+    return outer
+
+
+@jsonable_error(404, "Resource not found")
 def not_found(request):
     return render_to_response('error.html', {'error': '404'})
 
 
+@jsonable_error(500, "The Studio servers encountered an error")
 def server_error(request):
     return render_to_response('error.html', {'error': '500'})
 
 
+@jsonable_error(404, "Resource not found")
 def render_404(request):
     return HttpResponseNotFound(render_to_string('404.html', {}))
 
 
+@jsonable_error(500, "The Studio servers encountered an error")
 def render_500(request):
     return HttpResponseServerError(render_to_string('500.html', {}))