dependencies: Improve error message when variable is not found #17010

Job	Run time
Arch / PyPy	20m 12s
Arch Linux	20m 26s
CUDA (on Arch)	7m 41s
Fedora	17m 43s
Ubuntu Rolling (gcc, g++)	20m 13s
Gentoo	22m 12s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	13m 44s
OpenSUSE	17m 3s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	14m 49s
Ubuntu Bionic	15m 18s
Ubuntu Rolling (clang, clang++)	22m 19s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	15m 55s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	14m 30s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	13m 58s
	3h 56m 3s

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