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.
On each plugin initialization we create a separate python interpreter
which gets stored in the plugin_ctx. The main interpreter is
stored in py_ctx and is used for creating more interpreters (if more plugins
get loaded) and final python deinitialization.
The "traceback" module import and the ImportBlocker initialization was
moved, because it has to happen inside the plugin specific interpreters.
