Refactor: introduce abort_transaction in test_functions
TEST: add rollback_repo() to test_functions
TEST: return codes of standard repo management
FIX: transaction() fails with non-zero retval
FIX: abort() fails with non-zero retval
FIX: rollback() re-opens repo on failure
Merge pull request #532 from reneme/fix-non_zero_return

FIX: Some cvmfs_server Commands Failed with a Zero-Return Code