customerrors mode off not working on SharePoint 2010

During the developing of a custom field, I faced some errors, but the didn't work.
After some search, it seems that you have to do this trick on three locations. SharePoint 2010 has web.config files on the following locations:

• C:\inetpub\wwwroot\wss\VirtualDirectories\80\
• C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\CONFIG\
• C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\TEMPLATE\LAYOUTS

On all these locations you can find a web.config file.
Change customerrors to:

<customErrors mode="Off" />

and the SaveMode tag (if available) to:

<SafeMode MaxControls="200" CallStack="true" DirectFileDependencies="10" TotalFileDependencies="50" AllowPageLevelTrace="true">

as well as the compilation tag to:

<compilation batch="false" debug="true" optimizeCompilations="true">

This did the trick for me