Skip XML comments inside <node> and raise the passed error in handleError - #166

Fixes #161

With lxml, Node.load got XML comments as children whose tag is a function, so a comment was reported as an unknown scene node. The lxml parser in createElementTree now uses remove_comments=True and remove_pis=True, which covers every element, not only <node>. The stdlib parser already drops them.

That error then surfaced as RuntimeError: No active exception to reraise, because handleError used a bare raise while it is often called outside an except block. It now raises the error it was given.

Added test_collada_with_comments, which loads a document with a comment and a processing instruction inside <node>. With lxml it fails before the change; pytest collada/tests passes with and without lxml (60 tests).