Comment 1 for bug 1365759

Traceback:

Traceback (most recent call last):
  File "/usr/bin/apt-add-repository", line 163, in <module>
    if not sp.add_source_from_shortcut(shortcut, options.enable_source):
  File "/usr/lib/python3/dist-packages/softwareproperties/SoftwareProperties.py", line 750, in add_source_from_shortcut
    self.set_modified_sourceslist()
  File "/usr/lib/python3/dist-packages/softwareproperties/SoftwareProperties.py", line 484, in set_modified_sourceslist
    self.save_sourceslist()
  File "/usr/lib/python3/dist-packages/softwareproperties/SoftwareProperties.py", line 649, in save_sourceslist
    self.sourceslist.backup(".save")
  File "/usr/lib/python3/dist-packages/aptsources/sourceslist.py", line 383, in backup
    shutil.copy(source.file, "%s%s" % (source.file, backup_ext))
  File "/usr/lib/python3.4/shutil.py", line 228, in copy
    copyfile(src, dst, follow_symlinks=follow_symlinks)
  File "/usr/lib/python3.4/shutil.py", line 108, in copyfile
    with open(dst, 'wb') as fdst:
OSError: [Errno 30] Read-only file system: '/etc/apt/sources.list.save'