I just want a custom error page when a server error occurs - I've tried endless combinations of suggestions that I've found on google but nothing even begins to get me there.

Essentially, I have some error handling routines in my surface controllers where I want to log the error & then throw the exception so that somewhere down the line a 500 response code is returned & a custom error page is displayed.

                try

                {

                    repository.AddShoppingCartItem(sci);

                }

                catch(Exception e)

                {

                    Log.Error("whatever...");

                    throw;

                }

Then in my web.config I've tried

   <customErrors mode="On" defaultRedirect="error500">

   </customErrors>

and

   <customErrors mode="On" defaultRedirect="umbraco/surface/error500surface">

   </customErrors>

(where the 1st version uses a regular controller & the second a surface controller)

Neither do anything.

I've also added this line as others have suggested but it makes no difference:

What I get is my normal layout page getting rendered along with this message:

Error loading Partial View script (file: ~/Views/MacroPartials/Addcart.cshtml)

what I want to happen is for a custom error page to be displayed instead.

Reading all the stuff on google, I'm completely confused as to whether I need to configure Umbraco to handle these errors or whether it's a pure MVC approach. I get the impression that 404 errors are done through umbraco but 500 errors need to be a pure MVC approach. Don't know if anyone can confirm this. Either way, something up the chain is swallowing my throw statement & I don't know what it is.

I've also tried

return new HttpStatusCodeResult(500);

in place of the throw & that just gets me a nasty IIS error page.

Any help would be most appreciated as this is driving me nuts.

500 errors are server errors and no different in Umbraco than in regular IIS:

This is a server error and can only be solved by website admin who has access to files and the web-server. There can be one of/or multiple reasons to get this error. One has to track down the issue and handle accordingly.

http://www.codeproject.com/Articles/545054/HTTPplus-plus-e-plusInternalplusserverplus

More info:
http://serverfault.com/questions/407954/how-to-diagnose-a-500-internal-server-error-on-iis-7-5-when-nothing-is-written-t

Edit: apparently you may be able to enable custom errors like so:

http://helpnotes.vpasp.com/kb/611-General-hosting-questions/1089-How-to-Turn-Off-Friendly-Error-in-IIS-7xx/

Edit2: This may be of interest as well:

http://benfoster.io/blog/aspnet-mvc-custom-error-pages