Add exit code to ErrorReturnCode Exception

[NickHowden]

So that a generic handler can be written for ErrorReturnCode_N Exceptions, could the Exception constructor be modified to include the exit_code as an attribute.

i.e. in get_rc_exe

exc = type(name, (ErrorReturnCode, ), {exit_code:rc}

and

exc = type(name, (SignalException, ), {exit_code:rc}