fmt: fix error priority, make goal-errors more helpful

[BenWiederhake]

Fixes #6352.

In particular, this changes a deviation from GNU to a more helpful deviation from GNU:

$ ../gnu/src/fmt -g apple
../gnu/src/fmt: invalid width: 'apple'
[$? = 1]
$ cargo run -q fmt -g apple # Before this PR
fmt: invalid value 'apple'
[$? = 1]
$ cargo run -q fmt -g apple # After this PR
fmt: invalid goal: 'apple'
[$? = 1]

[BenWiederhake]

Changes since last push:

• Rebased
• Remove testcase of questionable use (thanks @cakebaker!)

In case anyone's curious, I was a bit paranoid about whether the value itself influences the priority in any way. This test proves that it does not; just as expected. That level of paranoia is not necessary with our own code.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/inotify-dir-recreate (fails in this run but passes in the 'main' branch)

[cakebaker]

Thanks!