How to delete all error messages in the Backtrace buffer so that when a new error occurs, instead of pre-recording with the old error, a new error is displayed.
The backtrace buffer has its own mode with several commands. I use the "q" key to kill the buffer and continue, but I'm sure others will work as well.
Here is a list of them: http://www.cs.utah.edu/dept/old/texinfo/emacs18/emacs_26.html#SEC190