Change schema issues

[monique2208]

trafficstars

It seems with the new validator some of the defined issue codes have changed. We previously used this list to find the issue codes, but it seems some of them are not on the list anymore and others have changed. Some of these we have been using to upgrade warnings that we find critical to errors (for example, missing events files).

I think a more extensive documentation of these codes is planned (#6), but do the changes in this list reflect changes to the validator? And if not, is there a more complete list somewhere that we can use in the meantime?