[transferred from bugzilla] use generic error codes
We should integrate generic error codes into our error domains to avoid the necessity to bump the minor API version in case a new error is needed.
This should be integrated whenever there is the next legitimate reason to increase the minor API version. [Which I hope to be 'never' except for pushing to 1.0.0]
The story behind this was:
In 0.6.6, I fixed a bug where the Updater did not find the rulesets file it downloaded from EFF. I decided that it would be the cleanest approach to add an error code for that. I falsely assumed that this was not worth bumping the api because i did not change any signatures. After asking several people on IRC it turned out it was indeed a bump-worthy change, so i did that when i published the next rulesets-update. This lead to a lot of confusion along depending upstream (epiphany) and downstream alike and should be avoided to happen ever again.