Don't retry failed TwilioSendSmsJob with error code 21211

[sauloperez]

Description

We're seeing a substantial number of errors in our error tracking due to exceptions like:

Twilio::REST::RestError Sidekiq/Decidim::ActionDelegator::TwilioSendSmsJob
[HTTP 400] 21211 : Unable to create record The 'To' number 6xxxxxxxx is not a valid phone number. https://www.twilio.com/docs/errors/21211

Admins keep uploading phone numbers without the country prefix (+34) and they are not valid for Twilio. This might be fine but given Sidekiq's retries the count in error tracking is showing an X times greater number than it actually is, while these retries will never work. They use a phone number that's proven to be invalid.

This needs to be solved at UX-level. We might need to make numbers without prefix work, but in the meantime, we shouldn't be wasting resources and messing up with error counts.

Proposed solution

We can make use of ActiveJob's discard_on checking for Twilio's error 21211 checking the RestError object.