errors.rb

DEFAULT_MAPPINGS = {
  not_present: "can't be empty".freeze,
  # …
}

ERROR_MARKUP =  '<p class="error-message">%s.</p>'.freeze

def error_message(form, field, **overrides)
  mappings = DEFAULT_MAPPINGS.merge(overrides)
  if form.errors[field].any?
    messages = form.errors[field].map { |code| mappings.fetch(code) }
    ERROR_MARKUP % messages.join(", ")
  end
end

# And then
# <%= error_message(form, :password, too_short: "must be at least 8 characters long") %>