[fix] Do not crash if archivemount is not there (#325)

src/yunohost/backup.py

@@ -1776,8 +1776,13 @@ class TarBackupMethod(BackupMethod):
         tar.close()
 
         # Mount the tarball
+        try:
             ret = subprocess.call(['archivemount', '-o', 'readonly',
                                    self._archive_file, self.work_dir])
+        except:
+            ret = -1
+
+        # If archivemount failed, extract the archive
         if ret != 0:
             logger.warning(m18n.n('backup_archive_mount_failed'))