AP_SerialManager: tweak error message #40960

Job	Run time
build (stm32f7, chibios, 10)	15m 34s
build (stm32h7, chibios, 10)	14m 8s
build (fmuv2-plane, chibios, 10)	3m 58s
build (periph-build, chibios, 10)	12m 37s
build (iofirmware, chibios, 10)	4m 4s
build (CubeOrange-bootloader, chibios, 10)	2m 57s
build (fmuv3-bootloader, chibios, 10)	3m 13s
build (revo-bootloader, chibios, 10)	3m 15s
build (stm32h7-debug, chibios, 10)	8m 55s
build (fmuv3, chibios, 10)	15m 14s
build (revo-mini, chibios, 10)	12m 12s
build (MatekF405-Wing, chibios, 10)	13m 2s
build (CubeOrange-ODID, chibios, 10)	4m 57s
build (CubeRedPrimary-bootloader, chibios, 10)	3m 7s
build (configure-all, chibios, 10)	12m 38s
build (build-options-defaults-test, chibios, 10)	26m 38s
build (signing, chibios, 10)	10m 0s
build (CubeOrange-PPP, chibios, 10)	6m 20s
build (SOHW, chibios, 10)	14m 51s
build (Pixhawk6X-PPPGW, chibios, 10)	3m 19s
build (new-check, chibios, 10)	14m 54s
	3h 25m 53s

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