Improve error message for generic subtyping. #2692

Job	Run time
Lint bash, docker, markdown, and yaml	1m 50s
Validate GNU Docker image builds	1m 44s
Validate musl Docker image builds	30s
Validate musl Docker release image builds	27s
Validate Windows Docker image builds	1m 22s
riscv64 Linux glibc	23m 44s
Validate GNU Docker release image builds	41s
Validate Windows Docker release image builds	2m 18s
Verify CHANGELOG is valid	6s
x86-64 Linux glibc	20m 51s
x86-64 Linux musl	22m 19s
aarch64 Linux glibc	23m 58s
armhf Linux glibc	27m 37s
arm Linux glibc	28m 12s
x86-64 Apple Darwin	27m 15s
arm64 Apple Darwin	16m 58s
x86-64 Windows MSVC	38m 16s
use dtrace	20m 43s
use pool_memalign	25m 3s
	4h 43m 54s

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