Sending errors are not reflected in the UI

[sbp]

When an email is sent via the web UI, the process function in server/endpoints/compose.py is responsible for sending the email by authenticating the environment and connecting to the MTA. It has several failure conditions which are marked by returning a dict with an "error" key. If one of these condition branches is taken, the user is not informed of the result. Instead it shows the following erroneous popup:

Message dispatched!
Your email has been sent. Depending on moderation rules, it may take a while before it shows up in the archives.

And the email is not actually sent.

[Humbedooh]

WDYT about making compose_send async and awaiting the result, check if there's an error, and use the standard modal then?