v0.18: Return soft error from archive-get when file is missing

* The archive-get function returns a 1 when the archive file is missing to differentiate from hard errors (ssh connection failure, file copy error, etc.)  This lets Postgres know that that the archive stream has terminated normally.  However, this does not take into account possible holes in the archive stream.
Merge branch 'master' of github.com:dwsteele/pg_backrest into dev