picture_pool: report when picture_pool_Wait was canceled

This way we can report to decoders, via the new decoder_WaitPicture() is the failure is recoverable (flushing) or not.

Fixes #26915 (closed) after using decoder_WaitPicture() in dav1d.

Deprecates !1893 (closed) and !1892 (closed) (and !1888 (closed) and !1889 (closed))