refactor: convert Bounce module to use neverthrow

[mantariksh]

Updates the Bounce module to use neverthrow.

A note on PR scope: there is a lot of refactoring that can be done in the Bounce module, but I don't think it should be a priority right now. The module is complex and was based on a long process of designing and refining, and refactoring it will not only take a long time but risks disrupting a service which has been automated for nearly a year. Hence I think a deeper refactor is out of scope of this PR, unless any part of it is genuinely un-readable.

Closes #193

Manual tests

• Create an email mode form. Add a valid collaborator with a registered contact number, and add a non-existent email as one of the email recipients. Submit the form. The form should NOT be deactivated, and the valid email recipient should not receive any emails/SMSes. Check the document in the Bounces collection corresponding to the form ID. The bounces array should contain the correct hasBounced key for each email addres.
• Remove all valid email recipients and leave only non-existent emails. Submit the form. The form should be deactivated, and the form admin + collaborators should receive ONE email and TWO smses each, informing them of the bounce and form deactivation.

[karrui]

lgtm, thanks for the effort