Handle internal server errors in the registration form

This fixes #1755: no error is shown on the client side when an attempt
to register an account receives a 500 Server Error response.

The failure() method in AuthController gets called and tries to do
{errors, reason} = response.data, but this raises a TypeError because
response.data is undefined (because the response didn't have a valid
JSON body), so the client crashes as well.

The fix is to catch this exception on the client side, and show a
generic error to the user.

h/static/scripts/account/auth-controller.coffee

@@ -18,7 +18,12 @@ class AuthController
       $scope.form?.$setPristine()
 
     failure = (form, response) ->
-      {errors, reason} = response.data
+      try
+        {errors, reason} = response.data
+      catch
+        form.responseErrorMessage = "Oops, something went wrong on the server.
+          Please try again later!"
+        return
       formRespond(form, errors, reason)
 
     this.submit = (form) ->