Make error messages more useful: add a URI

[Nico Williams]

Roland suggests:

[libdefaults]

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

where %E is whatever the error string would have been and %N is the
error code number (or symbolic name for it).

The default would be %E.

Additional contextual information could be provided with other tokens.

This would allow sites to use URIs or some other non-URI string if they
prefer.

Nico
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