plugins/python: add support for callback errstr arguments

Plugins can raise a sudo.PluginError exception to add context message
for the failure.

The callback's errstr gets filled up with the specified message.
But, as sudo expects a string constant (will not free the string),
we store it in the plugin context at least until next callback
invocation.

plugins/python/sudo_python_module.c

@@ -31,6 +31,7 @@ PyAPI_FUNC(PyObject *) PyStructSequence_GetItem(PyObject *, Py_ssize_t);
 
 // exceptions:
 PyObject *sudo_exc_SudoException;
+PyObject *sudo_exc_PluginError;
 static PyObject *sudo_exc_ConversationInterrupted;
 
 // the methods exposed in the "sudo" python module
@@ -562,6 +563,7 @@ sudo_module_init(void)
         } while(0);
 
     MODULE_ADD_EXCEPTION(SudoException, NULL);
+    MODULE_ADD_EXCEPTION(PluginError, NULL);
     MODULE_ADD_EXCEPTION(ConversationInterrupted, EXC_VAR(SudoException));
 
     #define MODULE_REGISTER_ENUM(name, key_values) \
@@ -611,6 +613,7 @@ cleanup:
     if (PyErr_Occurred()) {
         Py_CLEAR(py_module);
         Py_CLEAR(sudo_exc_SudoException);
+        Py_CLEAR(sudo_exc_PluginError);
         Py_CLEAR(sudo_exc_ConversationInterrupted);
     }