Add + use a db error callback function

Add + use a db error callback function
- Commit 7309f7498134f7b62f27624a4cdb91e1c6b82578 caused loss of
  some BDB error messages due misreading the docs: we used to explicitly
  set errfile to stderr through some hoops, and errcall to what
  always was NULL. But calling set_errcall() disables any output to
  the db errfile, and since we're no longer calling errfile...
- Removing the set_errcall() would restore output on stderr, but
  making it go through rpmlog() seems more consistent and lets
  API users catch it through rpmlog callback (in theory at least)