Add error.xml profile

This profile is for abstracting the semantics and links from the vnd.error media type for use in other formats.

I noticed in the [maze-alps](https://github.com/alps-io/profiles/blob/master/maze-alps.xml) profile, there was a `def` attribute used to point to documentation on the semantic or link. I went ahead and used this, though I'll be happy to change it if there is a more preferred way of doing so.