RFE: Warn on unmatched paths

[woky]

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/