Port over addition of hidden "Error: " to error messages

See alphagov/govuk-frontend#1221

shared-web/app/views/components/_govuk_error_message.html.slim

@@ -2,4 +2,8 @@
 / https://github.com/alphagov/govuk-frontend/
 /    blob/943ff14752f0a8a765ee3f90bc3e1ecd9205e36c/src/components/error-message/template.njk
 span.govuk-error-message *(local_assigns[:attributes] || {}) id=local_assigns[:id] class=local_assigns[:classes]
+  / Not using `.presence` or `||` here as we want to treat "" and nil differently.
+  - visually_hidden_text = local_assigns[:visuallyHiddenText].nil? ? "Error" : local_assigns[:visuallyHiddenText]
+  - if visually_hidden_text.present?
+    span.govuk-visually-hidden = "#{visually_hidden_text}: "
   = local_assigns[:html] || local_assigns[:text]