dependencies: Improve error message when variable is not found #17012

Job	Run time
Arch Linux	20m 8s
CUDA (on Arch)	8m 1s
Fedora	18m 16s
Gentoo	22m 33s
OpenSUSE	16m 51s
Ubuntu Bionic	15m 42s
Arch / PyPy	20m 40s
Ubuntu Rolling (gcc, g++)	21m 4s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	14m 17s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	14m 7s
Ubuntu Rolling (clang, clang++)	23m 22s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	16m 5s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	13m 55s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	14m 1s
	3h 59m 2s

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