posts: fix error on /posts?md5=<does_not_exist>.

app/controllers/posts_controller.rb

@@ -4,7 +4,7 @@ class PostsController < ApplicationController
 
   def index
     if params[:md5].present?
-      @post = Post.find_by_md5(params[:md5])
+      @post = Post.find_by!(md5: params[:md5])
       respond_with(@post) do |format|
         format.html { redirect_to(@post) }
       end

test/functional/posts_controller_test.rb

@@ -132,6 +132,11 @@ class PostsControllerTest < ActionDispatch::IntegrationTest
           get posts_path, params: { md5: @post.md5 }
           assert_redirected_to(@post)
         end
+
+        should "return error on nonexistent md5" do
+          get posts_path(md5: "foo")
+          assert_response 404
+        end
       end
 
       context "with a random search" do