[rubygems/rubygems] Improve error message when `path` and `gemspec` p… #605

Job	Run time
cargo test	57s
cargo clippy	39s
make (check, RUSTC='rustc +1.58.0', 1.58.0)	8m 15s
make (check, --enable-yjit=dev)	12m 32s
make (test-bundled-gems, --enable-yjit=dev)	25m 42s
make (yjit-bindgen, To fix: use patch in logs, --with-gcc=clang-14 --enable-yjit=dev, /usr/lib/ll...	4m 49s
make (check, --enable-yjit=dev, --yjit-call-threshold=1 --yjit-verify-ctx --yjit-code-gc, -T --yj...	17m 53s
YJIT Ubuntu result	2s
	1h 10m 49s

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