views: Raise 404 if downloading non-existent dependencies

views: Raise 404 if downloading non-existent dependencies

If a patch was processed by Patchwork before series support was added,
it will not have a series associated with it. As a result, it is not
possible to extract the dependencies for that patch from the series and
a 404 should be raised. This was not previously handled correctly.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Reviewed-by: Daniel Axtens <dja@axtens.net>
Reported-by: John McNamara <john.mcnamara@intel.com>
Fixes: e2dfd490 ("views: Add 'series' parameter to '/mbox' endpoint")
Closes: #189
(cherry picked from commit c975e20d3b5ba503df5fa1ea9f9a7490f3f223ae)