[cwconfig] improve assert error message. Single tuple element are processed in a non trivial way.

rqlrewrite: remove element in rewritten when we remove them from the select (closes #2236985) update _cleanup_inserted to avoid leaving rewritten variable behind when removing a snipset. insert_varmap_snippets was impacted too for unclear reason --- Before A KeyError was raised when: * multiple snipset is to be inserted on a statement * some *supported* snipset adds ambiguity (increase the number of solution) * some *unsupported* snipset adds new variable * The new variable require rewritting :: File "/home/pyves/src/fcw/cubicweb/rqlrewrite.py", line 185, in rewrite newsolutions = self.remove_ambiguities(snippets, newsolutions) File "/home/pyves/src/fcw/cubicweb/rqlrewrite.py", line 436, in remove_ambiguities variantes = self.build_variantes(newsolutions) File "/home/pyves/src/fcw/cubicweb/devtools/repotest.py", line 340, in _build_variantes variantes = _orig_build_variantes(self, newsolutions) File "/home/pyves/src/fcw/cubicweb/rqlrewrite.py", line 470, in build_variantes variante.append( (key, sol[newvar]) ) KeyError: u'D' This happen because the mechanism removing unsupported snipset does not remove entry in ``self.rewritten`` when it removes entry from ``self.select.defined_vars``. Iteration on ``self.rewritten`` then crash because values of ``rewritten`` are expected to be found in solution.

[js] fix $.fullCalendar.regional call (closes #2232028)