Fix "stg branch --delete" on a nonexistent branch

Before this patch, I get the following:

 $ stg branch --delete tmp
 Deleting branch "tmp" ... Traceback (most recent call last):
   File "/usr/bin/stg", line 43, in ?
     main()
   File "/var/lib/python-support/python2.4/stgit/main.py", line 281, in main
     command.func(parser, options, args)
   File "/var/lib/python-support/python2.4/stgit/commands/branch.py", line 190, in func
     __delete_branch(args[0], options.force)
   File "/var/lib/python-support/python2.4/stgit/commands/branch.py", line 100, in __delete_branch
     doomed.delete(force)
   File "/var/lib/python-support/python2.4/stgit/stack.py", line 758, in delete
     except GitException:
 NameError: global name 'GitException' is not defined

After it, I get:

 Deleting branch "tmp" ...
   Warning: Could not delete branch "tmp"
 done

Signed-off-by: Toby Allsopp <toby.allsopp@navman.co.nz>
Signed-off-by: Karl Hasselström <kha@treskal.com>