Add a file about committing
propagate translator function exceptions

The existing code tacitly uses the default translator
functions if a user-specified translator throws an
exception.  Propagate this exception instead so that
translator coders can know about them.
make Relation.replacejwithi operate on geometry like everyone else

While there is only one example of replacejwithi being called, it
is clear that the arguments are meant to be type Geometry, not
(Geometry, role) tuple.  So when replacing j with i, preserve the
existing role.