Improve handling of SMTP errors

Steps to reproduce

1. Compose a mail to 10 recipients, but forget a comma at the end of it or add some syntax error in to the TO field.
2. Send the email.

What happens?

• A notification shows up, with the error message's first 100 characters (out of maybe 1000), and you have no way of reading the rest of it as the notification doesn't allow you to do that. (afaik) "SMTP ERROR INVALID..." (I don't remember by head what was the message.)
• You may know about the outgoing folder, go there and open it, then you see no way of sending that again with corrected headers, thats frustrating.
• Then by accident you find the "Send again" in the outgoing list's long tap context menu and you are happy
  • And it repeats your signature every time you try it again, and you only notice that after you sent your mail to 10 person with 5 signatures under it:)

Propsed solution

• Send again into the outbox mail view context menu too
• A huge red notification in the outbox mail view with the error that blocked that mail's sending
• Format checking in the composer view on the fields, so that it would not be possible to send an email with incorrect recipients or anything in the first place

Environment

K-9 Mail version: 5.0.10

Android version: 6.0.1

Account type (IMAP, POP3, WebDAV/Exchange): imap

Thank you for K-9 mail, you are doing a great job out there!

Start by reproducing the SMTP failure in the composer and reviewing the outgoing mail view, focusing on the truncated error, resend entry point, and signature duplication described here. Done means the proposed outbox resend access, complete error display, and recipient validation are addressed or their scope is narrowed and verified on Android.