bob_dbmanage raises an exception if called w/o parameters

Here is the output:

$ ./bin/bob_dbmanage.py
Traceback (most recent call last):
  File "./bin/bob_dbmanage.py", line 23, in <module>
    sys.exit(bob.db.base.script.dbmanage.main())
  File "/Users/andre/Projects/bob/bob.db.base/bob/db/base/script/dbmanage.py", line 26, in main
    return args.func(args)
AttributeError: 'Namespace' object has no attribute 'func'

A help message or short usage message should be output in this case.