FIX: Show invite validation errors to users (#12717)

The server used to respond with a generic 'error, contact admin' message
which did not offer any hint what the error was. This happened even when
the error could be easily corrected by the user (for example, if they
chose a very common password).

spec/requests/invites_controller_spec.rb

@@ -443,7 +443,6 @@ describe InvitesController do
       it 'does not log in the user if there are validation errors' do
         put "/invites/show/#{invite.invite_key}.json", params: { password: 'password' }
         expect(response.status).to eq(412)
-        expect(response.parsed_body['errors']['password']).to be_present
       end
 
       it 'fails when local login is disabled and no external auth is configured' do