user deletions: fix error when given incorrect password.

Use validations instead of raising an exception when the password is
incorrect so that the controller can display errors sensibly.

Also fix users being logged out even when the deletion attempt failed
due to an incorrect password.

test/unit/user_deletion_test.rb

@@ -6,10 +6,8 @@ class UserDeletionTest < ActiveSupport::TestCase
       should "fail" do
         @user = create(:user)
         @deletion = UserDeletion.new(@user, "wrongpassword")
-
-        assert_raise(UserDeletion::ValidationError) do
-          @deletion.delete!
-        end
+        @deletion.delete!
+        assert_includes(@deletion.errors[:base], "Password is incorrect")
       end
     end
 
@@ -17,10 +15,8 @@ class UserDeletionTest < ActiveSupport::TestCase
       should "fail" do
         @user = create(:admin_user)
         @deletion = UserDeletion.new(@user, "password")
-
-        assert_raise(UserDeletion::ValidationError) do
-          @deletion.delete!
-        end
+        @deletion.delete!
+        assert_includes(@deletion.errors[:base], "Admins cannot delete their account")
       end
     end
   end