trace if no sources.list.d directory

[George Shuklin]

If no /etc/apt/sources.list.d directory exist, add-apt-repository traces:

 sudo add-apt-repository ppa:pjbroad/ppa

 More info: https://launchpad.net/~pjbroad/+archive/ubuntu/ppa
Press [ENTER] to continue or ctrl-c to cancel adding it

Traceback (most recent call last):
  File "/usr/bin/add-apt-repository", line 168, in <module>
    if not sp.add_source_from_shortcut(shortcut, options.enable_source):
  File "/usr/lib/python3/dist-packages/softwareproperties/SoftwareProperties.py", line 752, 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 650, in save_sourceslist
    self.sourceslist.save()
  File "/usr/lib/python3/dist-packages/aptsources/sourceslist.py", line 413, in save
    files[source.file] = open(source.file, "w")
FileNotFoundError: [Errno 2] No such file or directory: '/etc/apt/sources.list.d/pjbroad-ubuntu-ppa-xenial.list'
gpg: keyring `/tmp/tmpjea6zt0y/secring.gpg' created
gpg: keyring `/tmp/tmpjea6zt0y/pubring.gpg' created
gpg: requesting key DF90C47A from hkp server keyserver.ubuntu.com
gpg: /tmp/tmpjea6zt0y/trustdb.gpg: trustdb created
gpg: key DF90C47A: public key "Launchpad PPA for Paul Broadhead" imported
gpg: Total number processed: 1
gpg: imported: 1 (RSA: 1)
OK

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: software-properties-common 0.96.20
ProcVersionSignature: Ubuntu 4.4.0-18.34-generic 4.4.6
Uname: Linux 4.4.0-18-generic x86_64
ApportVersion: 2.20.1-0ubuntu2
Architecture: amd64
CurrentDesktop: X-Cinnamon
Date: Wed May 4 14:37:48 2016
EcryptfsInUse: Yes
InstallationDate: Installed on 2013-07-19 (1019 days ago)
InstallationMedia: Xubuntu 13.04 "Raring Ringtail" - Release i386 (20130423.1)
PackageArchitecture: all
SourcePackage: software-properties
UpgradeStatus: Upgraded to xenial on 2015-10-29 (187 days ago)