tahoe_backup.py: display warnings on errors instead of stopping the whole backup...

tahoe_backup.py: display warnings on errors instead of stopping the whole backup. Fix #729.

This patch displays a warning to the user in two cases:

  1. When special files like symlinks, fifos, devices, etc. are found in the
     local source.

  2. If files or directories are not readables by the user running the 'tahoe
     backup' command.

In verbose mode, the number of skipped files and directories is printed at the
end of the backup.

Exit status returned by 'tahoe backup':

  - 0 everything went fine
  - 1 the backup failed
  - 2 files were skipped during the backup