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/smtp.rs

@@ -36,8 +36,6 @@ pub enum Error {
     Oauth2Error { address: String },
     #[error("TLS error {0}")]
     Tls(#[from] async_native_tls::Error),
-    #[error("Sql {0}")]
-    Sql(#[from] crate::sql::Error),
     #[error("{0}")]
     Other(#[from] anyhow::Error),
 }