Is there way to test in genshi that certain get url parameter is set?
There seems to be no documentation to this.
I just need to render in template error message if there is parameter error in url.
Genshi is just the XHTML rendering engine behind Trac.
It would make sense to evaluate your GET request attributes in the IRequestHandler, that is preparing/providing data and template and passing on both for rendering and submission to the requesting client.
Furthermore use *add_notice* or *add_warning* from trac.web.chrome to present messages in Trac-like styling (the least intrusive way) to the user.