Interesting, didn't know that. I think the `os` / `net/http` bare style works well here--since the error messages are fairly clear (especially when combined with the var names), we can just avoid the duplication in the comments. 

I'd actually favor making the `ErrNotSupported` message clearer ("invalid command" seems opaque--why is it invalid?), but not sure what to use there (maybe just "unsupported command"?) or whether it breaks anything (it shouldn't; anything that cares should really be checking against the var, not the contents of the message).