bt2: raise when bt2.create_plugin_from_name() finds nothing

bt2: raise when bt2.create_plugin_from_name() finds nothing

Returning None from this function when there's no plugin is Pythonically
weird:

    plugin = bt2.create_plugin_from_name()
    print(plugin.name)

The execution should not reach print() here when the plugin is not
found.

Raise the new bt2.NoSuchPluginError exception instead.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>