One of the things I picked up from Dave Winer around 20 years ago is that errors messages should be of the form "Can't X because Y", and while I don't quite always use that exact grammar form, my errors pretty much all take that form when there isn't some other overriding local standard for errors. It's not a bad template. Most error messages will give you some hint about the Y, but the difference between "Error: file system full" and "Error: couldn't open log file '/var/log/myapp.log' because: file system full" is pretty substantial, especially if system state has mutated in the meantime and by the time you get to the system, none of the filesystems are full anymore.