[iso-core] Detect more offline errors when sending
[iso-core] Detect more offline errors when sending

Summary:
See title.
I really wish we could clean up this error handling a bit better, but I don't
think its super important right now.

Test Plan: manual

Reviewers: spang, evan, halla

Reviewed By: evan, halla

Differential Revision: https://phab.nylas.com/D3903

nirmit/nylas-mail,nylas/nylas-mail,nirmit/nylas-mail,nylas-mail-lives/nylas-mail,nylas/nylas-mail,simonft/nylas-mail,simonft/nylas-mail,simonft/nylas-mail,nylas/nylas-mail,nylas-mail-lives/nylas-mail,simonft/nylas-mail,nirmit/nylas-mail,nirmit/nylas-mail,nylas-mail-lives/nylas-mail,simonft/nylas-mail,nylas-mail-lives/nylas-mail,nylas/nylas-mail,nirmit/nylas-mail,nylas/nylas-mail,nylas-mail-lives/nylas-mail