Add a clear error message when a file from the worktree disappears during processing

Currently, Antora issues the following error message:

{ [Error: ENOENT: no such file or directory, lstat '/path/to/blog.yuzutech.fr/blog/modules/ROOT/pages/file.adoc~' in /path/to/blog.yuzutech.fr (ref: file <worktree> | path: blog)]
  errno: -2,
  code: 'ENOENT',
  syscall: 'lstat',
  path:
   '/path/to/blog.yuzutech.fr/blog/modules/ROOT/pages/file.adoc~' }

We should probably wrap the error to state clearly that the file has vanished during processing.