Issue #2501903 by pguillard, Manjit.Singh: inline form errors classnames to follow namestandard

core/modules/system/css/system.theme.css

core/modules/system/src/Tests/Form/FormTest.php

core/modules/system/src/Tests/Form/ValidationTest.php

core/modules/system/templates/form-element.html.twig

core/themes/bartik/css/components/form.css

core/themes/classy/templates/form/datetime-wrapper.html.twig

core/themes/classy/templates/form/fieldset.html.twig

core/themes/classy/templates/form/form-element.html.twig

core/themes/seven/css/components/form.css