apt-add-repository: Check that PPA supports currently running release before adding it

[Aminda Suomalainen]

I added WebKit PPA as suggested by text on Midori PPA page using the following command:

```
# apt-add-repository ppa:webkit-team
```

After running "aptitude update" I started to see 404 errors below, because WebKit PPA doesn't support Raring yet.

```
Err http://ppa.launchpad.net raring/main amd64 Packages
  404 Not Found
Err http://ppa.launchpad.net raring/main i386 Packages
  404 Not Found
Ign http://ppa.launchpad.net raring/main Translation-en
Ign http://ppa.launchpad.net raring/main Translation-fi
Ign http://ppa.launchpad.net raring/main Translation-en
Ign http://ppa.launchpad.net raring/main Translation-fi
97% [Working]W: Failed to fetch http://ppa.launchpad.net/webkit-team/ppa/ubuntu/dists/raring/main/binary-amd64/Packages: 404 Not Found
W: Failed to fetch http://ppa.launchpad.net/webkit-team/ppa/ubuntu/dists/raring/main/binary-i386/Packages: 404 Not Found
E: Some index files failed to download. They have been ignored, or old ones used instead.
E: Couldn't rebuild package cache
```

I think that apt-add-repository should first check that there are packages available in PPA for current release and if not, not add the repository or ask the user do they still want to add the repository.

----------

1) The release of Ubuntu you are using, via 'lsb_release -rd' or System -> About Ubuntu

Description: Ubuntu 13.04
Release: 13.04

2) The version of the package you are using, via 'apt-cache policy pkgname' or by checking in Software Center

software-properties-common:
  Installed: 0.92.17
  Candidate: 0.92.17
  Version table:
 *** 0.92.17 0
        500 mirror://mirrors.ubuntu.com/mirrors.txt/ raring/main amd64 Packages
        100 /var/lib/dpkg/status

3) What you expected to happen

I expected the PPA to work or getting warning about it not supporting my current release and question whether I still want to add the repository. I wouldn't mind even if it told me that it won't add that repository.

4) What happened instead

The PPA was added without any warnings and errors and after running "apt-get update" or "aptitude update" I started noticing 404 errors.

ProblemType: Bug
DistroRelease: Ubuntu 13.04
Package: software-properties-common 0.92.17
ProcVersionSignature: Ubuntu 3.8.0-19.29-generic 3.8.8
Uname: Linux 3.8.0-19-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.9.2-0ubuntu8
Architecture: amd64
Date: Sat Apr 27 10:43:34 2013
InstallationDate: Installed on 2013-04-24 (2 days ago)
InstallationMedia: Lubuntu 12.10 "Quantal Quetzal" - Release amd64 (20121017.1)
MarkForUpload: True
PackageArchitecture: all
SourcePackage: software-properties
UpgradeStatus: Upgraded to raring on 2013-04-26 (1 days ago)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.