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/oauth2.rs

@@ -309,7 +309,7 @@ impl Oauth2 {
     }
 }
 
-async fn is_expired(context: &Context) -> Result<bool, crate::sql::Error> {
+async fn is_expired(context: &Context) -> Result<bool> {
     let expire_timestamp = context
         .sql
         .get_raw_config_int64("oauth2_timestamp_expires")