Backport of "qtmux: Don't post an error message if pushing a sample failed with FLUSHING" into 1.20
Backport of !1711 (merged) "qtmux: Don't post an error message if pushing a sample failed with FLUSHING".
Commits:
- 8bda2ef4 qtmux: Don't post an error message if pushing a sample failed with FLUSHING