Fix exit on 404
Fix exit on 404

Most errors deserve a retry, but a notable exception is a missing
archive file.  This regressed because use of boto storage_uri was
deprecated when implementing multi-region support.

As-is, wal-e will spin in a tight loop when encountering such a
missing archive file without ever notifying Postgres that it is not
currently present.

DataDog/wal-e,ArtemZ/wal-e,x86Labs/wal-e,intoximeters/wal-e,modulexcite/wal-e,RichardKnop/wal-e,nagual13/wal-e,alex/wal-e,heroku/wal-e,fdr/wal-e,wal-e/wal-e,ajmarks/wal-e,tenstartups/wal-e,equa/wal-e