sort, uniq, fold: a read error does not say which input failed #15190

$ mkdir d
$ sort d
sort: read failed: Is a directory
$ uniq d
uniq: read error: Is a directory
$ fold d
fold: failed to read line: Is a directory

GNU 9.12 names the input, - for standard input: sort: read failed: d: Is a directory, uniq: error reading 'd': Is a directory, fold: d: Is a directory, and the same with - when the directory comes on standard input. With several inputs, uutils gives no way to tell which one failed. uutils main at a5bc606. Part of what misc/read-errors.sh in #14705 checks.