flatpak-utils-http: Ensure to wake up the main context on error

All the other code paths which set `data->done` or `data->error` ensure
to wake up the main context so its loop exit conditions can be checked
again — this code path was missing a wakeup though.

Spotted by code inspection; I haven’t hit this in the wild.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>
(cherry picked from commit 8ff02c0e5b0c2105e1b26a1a00ec264f6482ce3e)