Build marked success eventhough tagging is a failure. Log below
Subversion copy failed. svn: Commit failed (details follow):
svn: MKACTIVITY /isource/svnroot/SS_QNCSUP/!svn/act/497e7a64-3401-0010-98ef-5922a71ee1ba failed
Build step 'Perform Subversion tagging on successful build' marked build as failure
Notifying upstream projects of job completion
Yes, you could delegate the committer role.
The plugin extends hudson.tasks.Notifier that is triggered only after the build result is determined. My decision at that point was that the tagging failure shouldn't cause the build failure as it is not essential to the build artifacts so I chose the extension point, and I remember there wasn't more appropriate extension point to hook a plugin after a build completes but before finalizes the build status. It has been years since I wrote the plugin and Jenkins may have added more appropriate extension point by now, but I personally no longer use subversion and have no time to modify the plugin. If you're interested in to enhance this plugin, please let me know and I'll delegate the responsibility as a committer.
But we do have plugins, like junit, performance analysers, post build task, etc. that affect the state of the build after the analysis.
svn-tag is a post-build plugin, which is triggered by build success event and it cannot affect the status of the build even when tagging failed. It won't be fixed.