[rubygems/rubygems] Improve error message when `path` and `gemspec` p… #308

Job	Run time
cargo test	49s
make (check, --enable-yjit, --yjit)	5m 52s
make (check, --enable-yjit=dev, --yjit-call-threshold=1 --yjit-verify-ctx --yjit-code-gc, -T --yj...	8m 56s
YJIT macOS Arm64 result	2s
	15m 39s

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