bst2 does not propagate error to console when tracking returns 404

[nanonyme]

Failure Summary
    components/ladspa-sdk.bst:
    [17:08:49][00:00:03][88b09349][   track:components/ladspa-sdk.bst     ] FAILURE Background process died with error code 0
        Printing the last 80 lines from log file:
        /builds/freedesktop-sdk/freedesktop-sdk/cache/buildstream/logs/freedesktop-sdk/components-ladspa-sdk/88b09349-track.42.log
        ======================================================================
        BuildStream 1.95.2+0.g84f6d5e.dirty - Friday, 23-09-2022 at 17:08:46
        [--:--:--] START   [88b09349] components/ladspa-sdk.bst: Track
        [--:--:--] START   components/ladspa-sdk.bst: Tracking http://www.ladspa.org/download/ladspa_sdk_1.15.tgz
        [00:00:03] FAILURE components/ladspa-sdk.bst: Tracking http://www.ladspa.org/download/ladspa_sdk_1.15.tgz
        [00:00:03] FAILURE [88b09349] components/ladspa-sdk.bst: Background process died with error code 0

Upstream URL is apparently dead. bst2 must not in this case give up but instead fallback to mirror.

[nanonyme]

As per @gtristan there is behavioural change with how mirrors are dealt for tracking. But there is a bug in

buildstream/src/buildstream/downloadablefilesource.py

Lines 223 to 224 in a54f88b

	local_file, new_etag = self.blocking_activity(
	_download_file, (self.__get_urlopener(), self.url, etag, td), activity_name

which prevents user from seeing what the actual error was.

[nanonyme]

This is now probably fixed.