phabricator: don't abort if property writing fails during amending
Currently if one of the writediffproperty calls fails due to network issues
during the amending of commit messages to include the Diff. Rev. line, the
transaction is aborted and rolled back. This means that the associations
between the commits and the DREVs are lost for any already amended commits
because the removal of the local tags isn't covered by the rollback.
Differential Revision: https://phab.mercurial-scm.org/D6835