engines/io_uring_cmd: friendlier bad bs error msg #717

Job	Run time
build (linux-gcc)	10m 27s
build (linux-clang)	10m 52s
build (macos)	16m 17s
build (linux-i686-gcc)	8m 39s
build (android)	39s
build (windows-cygwin-64)	11m 53s
build (windows-cygwin-32)	10m 48s
build (windows-msys2-64)	13m 35s
build (android-recovery, ubuntu-22.04, aarch64-linux-android32)	41s
	1h 23m 51s

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