Make error messages more useful: add a URI

[Greg Hudson]

> [libdefaults]

>     err_fmt = %E. Please see https://whatever.example/%N

We would probably accept a patch for this, as long as it doesn't turn
out to be a lot of code.  There does need to be an internal API to get
the unadorned message for the places where we augment or copy error
messages, or an equivalent means of handling those cases without doubly
applying the err_fmt.