`cabal check`: clearly mark Errors (#8908)

* Fix typo

American spelling.

* Switch from `show` to `ppPackageCheck`

Show instance for `PackageCheck` is broken (not bijective with `read`) and
in the future will be deprecated.

* Add `warnError`

`warnError` is like `warn`, but instead of "Warning: …" prepends
"Error: …" to the message.  It is useful in those cases where you
want to highlight something is broken but do not want to stop the
program yet.

* cabal check: highlight error messages

Warnings that make `cabal check` fail (i.e. the package will be
refused by Hackage) are prepended by an "Error: " string, like
this:

Error: Instead of 'cc-options: -Llibdir' use 'extra-lib-dirs: libdir'

This commit also gets away from list comprehension.

* Make testsuite pass

`NoDupNames` is now caught with an `error` before check has the opportunity
to run.

* Add changelog for #8908

* Respect concordance

(Artem’s review) summary message will now say “The following errors…”
if the issues are “Error: …”s and “The following warnings…” otherwise.

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Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>