tumbler: Fix potential race condition between Ready and Error signals

As explained in the new comment in the Tumbler mixin, it is possible for
the Error and Ready signals from tumblerd to race if one thumbnailer
fails fast while another succeeds (for the same URI) slowly. This
happened for me with the cover and gst thumbnailers on
big_buck_bunny_smaller.ogv — the cover thumbnailer failed fast because
the search terms ‘big buck bunny smaller’ return nothing on omdbapi.com,
while the gst thumbnailer eventually succeeded by grabbing a frame from
the video.

Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D1299