postgresql: Don't raise StorageArgumentException in case of write conflicts

This should resolve a bunch of recurring Sentry issues:

• SWH-LOADER-SVN-1J
• SWH-LOADER-GIT-MJ
• SWH-LOADER-MERCURIAL-W
• SWH-LOADER-CORE-V4
• SWH-LOADER-GIT-KY
• SWH-LOADER-GIT-KQ

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Migrated from D6355 (view on Phabricator)