Provide better error message when MokManager is not found #905

Job	Run time
f41 x64 build compile_commands.json	2m 47s
f41 x64 build	2m 7s
f41 aa64 cross-build	2m 5s
f40 x64 build	2m 4s
f40 aa64 cross-build	1m 56s
f39 x64 build	2m 5s
f41 arm cross-build	1m 58s
centos9 x64 build	1m 11s
f40 arm cross-build	2m 22s
centos8 x64 build	1m 13s
f39 arm cross-build	2m 9s
f39 ia32 build	2m 3s
f41 x64 cross-build	2m 5s
centos8 ia32 build	1m 18s
f40 x64 cross-build	1m 53s
f39 x64 cross-build	1m 58s
f41 ia32 cross-build	2m 13s
f40 ia32 cross-build	2m 9s
f39 ia32 cross-build	2m 4s
	37m 40s

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