You can find the rough meaning of Mac OS status error numbers at:
-600 means “procNotFound”, proc being short for process.
By the looks of it you tried to quit an application and the system could not find the process for some reason (might even be that it had already been quit) and hence the error.
The /Users/peter/… just tells me the line number of file that the assertion occurred in (ie, the error happened on line 329 of the file KBMRunningApplication.mm). It is pure text, not any kind of actual reference to a file on your Mac (it’s just the path of a file on my Mac).