Fix type check error message grammar (fixes #20122)

compiler/GHC/Tc/Errors.hs

compiler/GHC/Utils/Outputable.hs

testsuite/tests/typecheck/should_fail/T20122.hs

testsuite/tests/typecheck/should_fail/T20122.stderr

testsuite/tests/typecheck/should_fail/T4921.stderr

testsuite/tests/typecheck/should_fail/T5858.stderr

testsuite/tests/typecheck/should_fail/all.T

testsuite/tests/typecheck/should_fail/tcfail040.stderr

testsuite/tests/typecheck/should_fail/tcfail043.stderr

testsuite/tests/typecheck/should_fail/tcfail128.stderr