Filter out error exit codes from non-pre scriptlets (rhbz#216221, #rhbz465409)

Filter out error exit codes from non-pre scriptlets (rhbz#216221, #rhbz465409)
- typos and thinkos in scriptlets often cause duplicates to be left around
  for no good reason
- treat non-zero exit codes from scriptlets as just warnings instead of
  errors
- ... except that %pre and %preun are sometimes used for preventing
  install/uninstall of a package, preserve the traditional behavior for them
- abuse rpmtsNotify() "total" value for signaling warning/error to callback