Fix SIGABRT when python subcommand raises an exception
Fix SIGABRT when python subcommand raises an exception

Before, `ledger python -- -c 'raise RuntimeError'` would terminate
messily via SIGABRT, printing the following:

    terminate called after throwing an instance of 'int'
    [1]    2151711 abort (core dumped)  ledger python -c 'raise RuntimeError'

This change makes the python subcommand throw a standard C++ exception
rather than just a plain int, which is never caught and triggers the
SIGABRT.  Now, the process prints the uncaught Python exception as
usual and then exits with exit code 1.

afh/ledger,ledger/ledger,afh/ledger,ledger/ledger,ledger/ledger,ledger/ledger,ledger/ledger,afh/ledger,afh/ledger,afh/ledger