LG-543 Retry failed Twilio Verify requests

[monfresh]

Why: Sometimes, requests to the Twilio Verify service fail to
connect or time out. Twilio suggested that we retry the request to see
if it works the second time. We had implemented retries for the Twilio
REST API earlier. Now, we are adding it to the Verify service as well.

How:

• Use Faraday to catch and rescue timeouts and connection failures, and
  raise our own custom VerifyError. Typhoeus does not raise exceptions,
  and therefore does not support retries out of the box. Since the
  twilio-ruby gem uses Faraday, and we rescue Faraday errors in
  TwilioService, I thought we'd use Faraday here too for consistency.
• Use dependency injection to specify the connection class, which makes
  testing easier.
• Include "Twilio Verify" in the custom error message to make it easier
  to differentiate from timeout and failed connection errors raised by
  the Twilio REST service.

Hi! Before submitting your PR for review, and/or before merging it, please
go through the checklists below. These represent the more critical elements
of our code quality guidelines. The rest of the list can be found in
CONTRIBUTING.md

• When adding a new controller that requires the user to be fully
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• Indexes were added if necessary. This article provides a good overview
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• Verified that the changes don't affect other apps (such as the dashboard)

• When relevant, a rake task is created to populate the necessary DB columns
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  who might not have interacted with this column yet (such as users who have not
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  issues. In that case, all the migration did was add a new column and an index to
  the Users table, which might seem innocuous.

• The changes are compatible with data that was encrypted with the old code.

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• When adding user data to the session, use the user_session helper
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• Tests added for this feature/bug

• Prefer feature/integration specs over controller specs

• When adding code that reads data, write tests for nil values, empty strings,
  and invalid inputs.

[monfresh]

To match our Rubocop setting. Code Climate was complaining that send_sms was too complex. I'm not sure how to make it simpler. It seems readable to me. Thoughts?

[stevegsa]

Shouldn't this be 'tries = 2'?

[monfresh]

Nope. It would never get to zero in that case and would keep retrying infinitely.

[stevegsa]

Ah retry

[stevegsa]

LGTM