Simplify SQL error handling (#2415)

* Remove sql::error submodule

Use anyhow errors instead.

* Remove explicit checks for open SQL connection

An error will be thrown anyway during attempt to execute query.

* Don't use `with_conn()` and remove it

* Remove unused `with_conn_async`

* Resultify markseen_msgs

src/sql/migrations.rs

@@ -1,10 +1,12 @@
-use super::{Result, Sql};
+use anyhow::Result;
+
 use crate::config::Config;
 use crate::constants::ShowEmails;
 use crate::context::Context;
 use crate::dc_tools::EmailAddress;
 use crate::imap;
 use crate::provider::get_provider_by_domain;
+use crate::sql::Sql;
 
 const DBVERSION: i32 = 68;
 const VERSION_CFG: &str = "dbversion";