more direct way of handling Future success/fail

[tomholub]

It's similarly weird to use map if we are talking about a single Future executing one time with a single failure or success. Map implies working with array of potentially many items, which seems a mismatch for what we're doing (even if this is otherwise common in iOS development).

I'll merge this but file a separate issue.

Originally posted by @tomholub in #410 (comment)

 return getPubKeys(for: recipients)
 .mapError { ComposeMessageError.validationError($0) }
 .map { allRecipientPubs in
 let replyToMimeMsg = input.replyToMime
 .flatMap { String(data: $0, encoding: .utf8) }

 return SendableMsg(
 text: text,
 to: recipients.map(\.email),
 cc: [],
 bcc: [],
 from: email,
 subject: subject,
 replyToMimeMsg: replyToMimeMsg,
 atts: atts,
 pubKeys: allRecipientPubs + [myPubKey]
 )
 }
 }

[tomholub]

nevermind, transitioning to async/await