Fix SandboxTest.setUp() in case gh.git_repo() fails with a server error

If `gh.git_repo()` fails due to e.g. a server error, this commit ensures we return to the initial folder instead of staying in a deleted folder

To test this PR:
see http://hudson.openmicroscopy.org.uk/view/Failing/job/SCC-self-merge/142/console
we shouldn't get any
```
======================================================================
ERROR: testDeployInitBrokenSymlink (test.unit.TestDeploy.TestDeploy)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/omero/slave/workspace/SCC-self-merge/snoopycrimecop/test/unit/TestDeploy.py", line 33, in setUp
    self.folder = os.path.abspath("deploy_test")
  File "/home/omero/slave/workspace/SCC-self-merge/virtualenv/lib/python2.6/posixpath.py", line 343, in abspath
    cwd = os.getcwd()
OSError: [Errno 2] No such file or directory

```
error if SandboxTest.setUp()` fails