Fix collabora thumbex example

This randomly came to my mind but I can't do it myself now.

When handling exceptions in Wrapper.seek() I kept the generator and underlying file object opened. I didn't check but I guess Request doesn't automatically stops a generator passed to it.

Something like:

except Exception as e:
    if isinstance(e, HTTPError):
        e.close()
    self.close()
    raise e

and in close():

if self.response:
    self.response.close()
self.request.data.close()

Not a big deal because garbage-collected generator and file should be closed by runtime. But still explicit is better than implicit.