bootctl: check if files specified by boot entry exist, and warn if not

bootctl: check if files specified by boot entry exist, and warn if not

Example output:
        title: Fedora 30 (Workstation Edition) (5.0.5-300.fc30.x86_64)
           id: 08a5690a2eed47cf92ac0a5d2e3cf6b0-5.0.5-bad-300.fc30.x86_64
       source: /boot/efi/loader/entries/08a5690a2eed47cf92ac0a5d2e3cf6b0-5.0.5-bad-300.fc30.x86_64.conf
      version: 5.0.5-300.fc30.x86_64
   machine-id: 08a5690a2eed47cf92ac0a5d2e3cf6b0
        linux: /08a5690a2eed47cf92ac0a/5.0.5-300.fc30.x86_64/linux (No such file or directory)
       initrd: /08a5690a2eed47cf92ac0a/5.0.5-300.fc30.x86_64/initrd (No such file or directory)
               /08a5690a2eed47cf92ac0a/5.0.5-300.fc30.x86_64/initrd2 (No such file or directory)
      options: ...