[contactsd] Do not fail sync due to removing non-existent contact

If are contact that we intend to remove is not found in the export
database, continue without the removal.
[contactsd] Do not abort export sync if updates are applied

The export plugin does not apply changes transactionally; we should
not abort a sync if some changes have been applied, since partial
changes will then accumulate.

If add/modify changes are successfully applied, then do not abort
sync because removals fail, or because the self contact cannot be
updated.