RFE: Warn on unmatched paths

When some paths in `contents` do not match any files in the package, chisel silently ignores them. In the example below I slice `/lib` from `base-files` which in the package is actually a directory (`/lib/`, with slash at the end, would work) but I expected it to be a symlink. I should be told that the path didn't match anything.

```
$ cat release/slices/base-files.yaml
package: base-files
slices:
lib:
contents:
/lib:
/nonexistent:
$ rm -rf output/ && mkdir output
$ chisel cut --release release/ --root output/ base-files_lib
...
2022/06/02 10:54:23 Extracting files from package "base-files"...
$ find output/
output/
```