<rdar://problem/14266411>

The semi-unofficial way of returning a status from a Python command was to return a string (e.g. return "no such variable was found") that LLDB would pick as a clue of an error having happened

This checkin changes that:
- SBCommandReturnObject now exports a SetError() call, which can take an SBError or a plain C-string
- script commands now drop any return value and expect the SBCommandReturnObject ("return object") to be filled in appropriately - if you do nothing, a success will be assumed

If your commands were relying on returning a value and having LLDB pick that up as an error, please change your commands to SetError() through the return object or expect changes in behavior