[rubygems/rubygems] Improve error message when `path` and `gemspec` p… #701

Job	Run time
make (check, --enable-shared --enable-load-relative)	8m 18s
make (check, ubuntu-24.04)	8m 57s
make (test-bundler-parallel, 50)	28m 50s
make (check, --disable-yjit)	7m 5s
make (check, i686)	11m 7s
make (check, cppflags=-DVM_CHECK_MODE)	9m 4s
make (test-bundled-gems)	20m 29s
Ubuntu result	3s
	1h 33m 53s

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