/users/new: show error message when account signup fails.

app/controllers/users_controller.rb

@@ -47,6 +47,8 @@ class UsersController < ApplicationController
       @user.save
       if @user.errors.empty?
         session[:user_id] = @user.id
+      else
+        flash[:notice] = "Sign up failed: #{@user.errors.full_messages.join("; ")}"
       end
       set_current_user
       respond_with(@user)