Big rewrite of the OS-related functions

Currently the OSes have a special, customized error handling: the OS
object can represent either a valid OS, or an invalid OS. The associated
function, instead of raising other exception or failing, create custom
OS objects representing failed OSes.

While this was good when no other RPC had failure handling, it's
extremely different from how other function in backend.py expect
failures to be signalled.

This patch reworks this completely:
  - the OS object always represents valid OSes (the next patch will
    remove the valid/invalid field and associated constants)
  - the call_os_diagnose returns instead of a list of OS objects, a list
    of (name, path, status, diagnose_msg); the status is then used in
    cmdlib to determine validity and the status and diagnose_msg values
    are used in gnt-os for display
  - call_os_get returns either a valid OS or a RPC remote failure (with
    the error message)
  - the other functions in backend.py now just call backend.OSFromDisk()
    which will return either a valid OS object or raise an exception
  - the bulk of the OSFromDisk was moved to _TryOSFromDisk which returns
    status, value for the functions which don't want an exception raised

The gnt-os list and diagnose commands still work after this patch.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>