catch github checkout errors

If github is down our jobs fail with the below error. We could catch this and make it abort instead of fail so it doesn't email people.
Building remotely on 64.91.254.123 in workspace <http://jenkins.ros.org/job/ros-hydro-turtlebot_binarydeb_precise_amd64/ws/>
Checkout:ros-hydro-turtlebot_binarydeb_precise_amd64 / <http://jenkins.ros.org/job/ros-hydro-turtlebot_binarydeb_precise_amd64/ws/> - hudson.remoting.Channel@4b16c641:64.91.254.123
Using strategy: Default
Last Built Revision: Revision 6e36bdae0f3df20b056cf7fb552f2d06847acce0 (origin/master)
Cloning the remote Git repository
Cloning repository git://github.com/ros-infrastructure/buildfarm.git
git --version
git version 1.7.9.5
ERROR: Error cloning remote repo 'origin' : Could not clone git://github.com/ros-infrastructure/buildfarm.git
hudson.plugins.git.GitException: Could not clone git://github.com/ros-infrastructure/buildfarm.git
        at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.clone(CliGitAPIImpl.java:220)
        at hudson.plugins.git.GitSCM$2.invoke(GitSCM.java:1001)
        at hudson.plugins.git.GitSCM$2.invoke(GitSCM.java:942)
        at hudson.FilePath$FileCallableWrapper.call(FilePath.java:2393)
        at hudson.remoting.UserRequest.perform(UserRequest.java:118)
        at hudson.remoting.UserRequest.perform(UserRequest.java:48)
        at hudson.remoting.Request$2.run(Request.java:326)
        at hudson.remoting.InterceptingExecutorService$1.call(InterceptingExecutorService.java:72)
        at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334)
        at java.util.concurrent.FutureTask.run(FutureTask.java:166)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1146)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
        at java.lang.Thread.run(Thread.java:679)
Caused by: hudson.plugins.git.GitException: Command "git clone --progress -o origin git://github.com/ros-infrastructure/buildfarm.git <http://jenkins.ros.org/job/ros-hydro-turtlebot_binarydeb_precise_amd64/ws/monitored_vcs"> returned status code 128:
stdout: Cloning into '<http://jenkins.ros.org/job/ros-hydro-turtlebot_binarydeb_precise_amd64/ws/monitored_vcs'...>

stderr: fatal: unable to connect to github.com:
github.com[0: 204.232.175.90]: errno=Connection timed out

        at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.launchCommandIn(CliGitAPIImpl.java:774)
        at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.clone(CliGitAPIImpl.java:218)
        ... 12 more
Trying next repository
ERROR: Could not clone repository
FATAL: Could not clone
hudson.plugins.git.GitException: Could not clone
        at hudson.plugins.git.GitSCM$2.invoke(GitSCM.java:1013)
        at hudson.plugins.git.GitSCM$2.invoke(GitSCM.java:942)
        at hudson.FilePath$FileCallableWrapper.call(FilePath.java:2393)
        at hudson.remoting.UserRequest.perform(UserRequest.java:118)
        at hudson.remoting.UserRequest.perform(UserRequest.java:48)
        at hudson.remoting.Request$2.run(Request.java:326)
        at hudson.remoting.InterceptingExecutorService$1.call(InterceptingExecutorService.java:72)
        at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334)
        at java.util.concurrent.FutureTask.run(FutureTask.java:166)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1146)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
        at java.lang.Thread.run(Thread.java:679)
ros-infrastructure / buildfarm

catch github checkout errors #139
