How Do I find Out the Real Error When I get an error screen with just a correlation id ?

Hi

In SP2010 and SP2013 if you get an error with a correlation ID you can use tools like ULSViewer to find out what the real error is and can then fix it.  There does do seem to be a corresponding tool in SharePoint Online.

How can I find out what caused my error from the Correlation ID ?  I am not using any C# or JavaScript code.  I am just trying to change a masterpage for a site from masterpage A to masterpage B so I cannot put in any debugging code.

This is very frustrating - I cannot believe that such a tool is not available.

Thanks

Nigel