refactor(crypto): minor refactor of decryptWithAttachments

[seaerchin]

Problem

Refactored decryptWithAttachments for additional clarity

Issues faced

1. lack of chaining means that everything has to be within a try/catch block

[mantariksh]

out of curiosity, why is this line necessary?

[seaerchin]

it isn't! i actually left it in because i thought it was convention @__@ i'll remove it!

[seaerchin]

i went to investigate more and it was q enlightening for me! the reason why this is done is because it's an error. when js (and by extension, ts) throws an error, the error printed to the output is done using error.name + ' ' + error.message - this means that when the error here is thrown, because it's missing the name property, it will default to Error (as it calls super(message)). While the argument might be that it's guaranteed to never be thrown, i think this is a relatively small change that helps in the future so i'd just leave it in!