Improve imap_append_message() error message handling

Improve imap_append_message() error message handling

If the rc is IMAP_CMD_BAD, then either idata->buf is stale or an error
message has already been printed (in cmd_handle_untagged()).

Use imap_next_word() to skip over the next two words instead of
directly skipping over SEQLEN, in case the buffer is in a different
format.  We don't want to jump over the end of string.

Skip the mutt_error() if there is nothing to print.

Co-authored-by: Richard Russon <rich@flatcap.org>