Distinguish between invalid username and taken username (fixes #199)

app/controllers/usernames_controller.rb

@@ -12,7 +12,7 @@ class UsernamesController < ApplicationController
     if @user.update(username: params[:user][:username].strip)
       redirect_to_profile(@user)
     else
-      @invalid_username = true
+      @error = @user.errors[:username].to_s =~ /taken/ ? :username_taken : :username_invalid
       render :new
     end
   end

app/views/usernames/new.html.slim

@@ -16,12 +16,16 @@
         .form-group
           = f.label :username, 'Your username:'
           = f.text_field :username, class: 'form-control', 'data-behavior' => 'focus'
-          - if @invalid_username
+          - if @error == :username_invalid
             br
             p.text-danger
               ' Use only letters, digits and "-" character.
               ' Examples:
               em johnny-knoxville, destroyer666
+          - if @error == :username_taken
+            br
+            p.text-danger
+              ' This username was already taken.
         .form-group
           = f.submit 'Continue', class: 'btn btn-primary'
           a.btn href=skip_username_path I'll do it later