Report a clearer error when .gcda files are found without .gcno files

[chiliec]

Fixes #586.

When the inputs contain .gcda files but no .gcno files (e.g. after find . -name "*.gc*" -delete wiped the compile-time notes), grcov panicked with the generic No input files found. It now reports:

Invalid input files: found .gcda files without .gcno files (the .gcno files should be produced during compilation, were they removed?)

The existing message is kept when there are no input files at all.

Test: tightened the existing test_zip_producer_with_gcda_archive_and_no_gcno_archive to #[should_panic(expected = "found .gcda files without .gcno files")]. It fails on main (panic has the old message) and passes with this change.

Validation (local, rustc 1.98.1):

• cargo test --lib: 137 passed. 4 llvm_tools::tests::* fail because llvm-profdata isn't installed on my machine. They fail the same way on unmodified main.
• cargo fmt --check: clean
• cargo clippy --lib --tests: no warnings