This element is optional. Without it, ClickOnce displays an error dialog box showing the exception stack. If the customErrorReporting element is present, ClickOnce will instead display the URI indicated by the uri parameter. The target URI will include the outer exception class, the inner exception class, and the inner exception message as parameters.

Use this element to add error reporting functionality to your application. Since the generated URI includes information about the type of error, your Web site can parse that information and display, for example, an appropriate troubleshooting screen.

The following snippet shows the customErrorReporting element, together with the generated URI it might produce.

<customErrorReporting uri=http://www.contoso.com/applications/error.asp />

Example Generated Error:
http://www.contoso.com/applications/error.asp? outer=System.Deployment.Application.InvalidDeploymentException&&inner=System.Deployment.Application.InvalidDeploymentException&&msg=The%20application%20manifest%20is%20signed,%20but%20the%20deployment%20manifest%20is%20unsigned.%20Both%20manifests%20must%20be%20either%20signed%20or%20unsigned.

Reference