lib/smbconf: add a python function for raising smbconf exceptions

lib/smbconf: add a python function for raising smbconf exceptions

The previous implementation in C was private to the module. Add
a small python wrapper function so that a different python module
may reuse the implementation.

The python level function is prefixed with "_" to mark it as
"private". Only future cooperating modules in the samba sources
should make use of it.

The function is shared at the python level as per the recommendation:
    https://stackoverflow.com/a/2136670

Signed-off-by: John Mulligan <jmulligan@redhat.com>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>