The plugins libdnf_plugin_new_instance and dnf5_plugin_new_instance API functions discard exceptions

[jrohel]

The libdnf_plugin_new_instance and dnf5_plugin_new_instance functions return a null pointer if the instantiation of the plugin fails. The exception with failure information is discarded. Thus, the caller (libdnf library or dnf5 application) will not know the reason for the failure.

The above functions are linked as "C" functions. It is necessary to provide caller access to the exception.

[ppisar]

I confirm the libdnf_plugin_new_instance functions as defined e.g. in libdnf5-plugins/actions/actions.cpp translates the exception to returning nullptr. If we want to return an error message, we need to change the prototype: either add a new argument or change the return type.

[ppisar]

To prevent from the API/ABI break we could add a new function and keep to old one.

[jrohel]

I have extended the dnf5 and libdnf plugin API with a new function to get the last exception. The original functions are unchanged. Plugins that do not implement the new functions can still be used. API and ABI remain compatible.

#2052