Allow for runpy returning non-integer results. (#65)

* Allow for runpy returning non-integer results.

* Rework exit path for SystemExit.

* Ensure no extra output for a SystemExit with error string.

* Handle the case of SystemExit()/SystemExit(None).

{{ cookiecutter.format }}/{{ cookiecutter.class_name }}/main.m

@@ -247,23 +247,36 @@ int main(int argc, char *argv[]) {
                     exit(-5);
                 }
 
+                traceback_str = NULL;
                 if (PyErr_GivenExceptionMatches(exc_value, PyExc_SystemExit)) {
                     systemExit_code = PyObject_GetAttrString(exc_value, "code");
                     if (systemExit_code == NULL) {
-                        debug_log(@"Could not determine exit code");
+                        traceback_str = @"Could not determine exit code";
                         ret = -10;
-                    }
-                    else {
+                    } else if (systemExit_code == Py_None) {
+                        // SystemExit with a code of None; documented as a
+                        // return code of 0.
+                        ret = 0;
+                    } else if (PyLong_Check(systemExit_code)) {
+                        // SystemExit with error code
                         ret = (int) PyLong_AsLong(systemExit_code);
+                    } else {
+                        // Any other SystemExit value - convert to a string, and
+                        // use the string as the traceback, and use the
+                        // documented SystemExit return value of 1.
+                        ret = 1;
+                        traceback_str = [NSString stringWithUTF8String:PyUnicode_AsUTF8(PyObject_Str(systemExit_code))];
                     }
                 } else {
                     // Non-SystemExit; likely an uncaught exception
-                    ret = -6;
                     info_log(@"---------------------------------------------------------------------------");
                     info_log(@"Application quit abnormally!");
+                    ret = -6;
+                    traceback_str = format_traceback(exc_type, exc_value, exc_traceback);
+                }
 
+                if (traceback_str != NULL) {
                     // Display stack trace in the crash dialog.
-                    traceback_str = format_traceback(exc_type, exc_value, exc_traceback);
                     crash_dialog(traceback_str);
                 }
             }