The `structerr.New()` constructor returns errors with an `Internal`
code. This makes for a somewhat weird interface though: `New()` shall be
used in cases where the actual error condition is not quite clear, but
by already setting an `Internal` error code we'll ultimately propagate
that code even if an ancestor in the callchain would know better.

Adjust the function to instead return `Unknown` errors. Together with
the change to always override `Unknown` errors with more specific ones,
this makes for a new calling convention of how to construct errors:
always use `New()`, unless you know the actual root cause. Like this,
the first caller that knows more about the root cause will override the
`Unknown` error.

Document this calling convention.