I filed https://todo.sr.ht/~sircmpwn/aerc2/199 because I was confused by an error message when I tried to send an email. Apparently, that email was sent successfully but then failed to copy to a nonexistent Sent folder. If the error had included the name of the folder, I would have had a lot more context about what happened.
I wish the error said
PqOjfQ NO [TRYCREATE] FolderSentdoesn't exist. (Failure)
And similarly the UI-visible error could say
FolderSentdoesn't exist. (Failure)