When my WebApp crashed, the user see a yellow/black window:
“This application has encountered an error and cannot continue.” with some data and a comment field.
My users told me, they send some comments with the errors. But where can I read the errors and comments?
Add the JavaScriptError event to your session object.
In that event you can save the error and user details(the comments) to a database/text file or something .
But, by default there should be a file called Errors.log in your app folder with the errors and user details though so take a look there 
Thank you. I have to search for the errors.log file.
It should be in your app root folder.
The log is not created in a debug app run within the IDE.