BURs: clean up parsing and error handling.

* Don't raise exceptions when a BUR is invalid. Instead, use Rails
  validations to return errors. Fixes invalid BURs potentially raising
  exceptions in views. Also makes it so that each error in a BUR is
  reported, not just the first one.

* Revalidate the BUR whenever the script is edited, not just when the
  BUR is created. Ensures the BUR can't be broken by editing. Fixes a bug
  where forum threads could be broken by someone editing a BUR and
  breaking the syntax, thereby causing the BUR to raise an unparseable
  script error when the forum thread was viewed.

* Validate that removed aliases and implication actually exist.

* Validate that the tag actually exists when changing a tag's category.

* Combine bulk update request processor unit tests with main bulk update
  request unit tests.

test/functional/bulk_update_requests_controller_test.rb

@@ -7,7 +7,7 @@ class BulkUpdateRequestsControllerTest < ActionDispatch::IntegrationTest
       @builder = create(:builder_user)
       @admin = create(:admin_user)
       as(@admin) { @forum_topic = create(:forum_topic, id: 100, category_id: 0) }
-      as(@user) { @bulk_update_request = create(:bulk_update_request, user: @user, forum_topic: @forum_topic) }
+      as(@user) { @bulk_update_request = create(:bulk_update_request, user: @user, forum_topic: @forum_topic, script: "create alias aaa -> bbb") }
     end
 
     context "#new" do
@@ -71,6 +71,12 @@ class BulkUpdateRequestsControllerTest < ActionDispatch::IntegrationTest
         assert_response 403
         assert_equal("create alias aaa -> bbb", @bulk_update_request.reload.script)
       end
+
+      should "fail for an invalid script" do
+        put_auth bulk_update_request_path(@bulk_update_request.id), @user, params: { bulk_update_request: { script: "create alis gray -> grey" }}
+        assert_response :success
+        assert_equal("create alias aaa -> bbb", @bulk_update_request.reload.script)
+      end
     end
 
     context "#index" do