Provide better error message when MokManager is not found #906

Job	Run time
f41 x64 build compile_commands.json	2m 45s
f41 x64 build	2m 9s
f41 aa64 cross-build	2m 6s
f40 x64 build	2m 4s
f40 aa64 cross-build	1m 57s
f41 arm cross-build	1m 56s
f39 x64 build	2m 3s
f40 arm cross-build	1m 57s
centos9 x64 build	1m 9s
f39 arm cross-build	2m 5s
centos8 x64 build	1m 14s
f39 ia32 build	2m 12s
f41 x64 cross-build	2m 0s
f40 x64 cross-build	1m 51s
centos8 ia32 build	1m 19s
f39 x64 cross-build	1m 59s
f41 ia32 cross-build	2m 4s
f40 ia32 cross-build	1m 58s
f39 ia32 cross-build	1m 57s
	36m 45s

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