_form.html.erb

<% @user.errors[:login].each do |err| %>
  <span class="help-block">
    <p><%= error_message(err) %></p>
  </end>
<% end%>

user.rb

class User < ActiveRecord::Base
  validates :login,
    length: {
      in: 2..10,
      message: {
        code: :INVALID_LENGTH,
        max: 10,
        min: 2,
        description: "Invalid length"
    }
  }
end

users_helper.rb

module UsersHelper

  def error_message(error)
    return error if error.is_a?(String)
    method = error[:code].downcase + "_error_message"
    return send(method, error) if respond_to?(method) # We have a special way of displaying such error
    return error[:description]                 # Just display the description from model
  end

  def invalid_length_error_message(error)
    "length of this text is invalid. Minimum is <strong>#{error[:min]}</strong> and maximum is <strong>#{error[:max]}</strong>".html_safe
  end

end