feat(mysql): thread E: Execute through the executor

+ handle normal errors separate from unexpected errors, an unexpected
   error causes the connection to close (in which case, if this was
   behind a pool, the pool would not allow this connection to be
   acquired again)

sqlx-core/src/error.rs

@@ -30,6 +30,8 @@ pub enum Error {
     ///
     RowNotFound,
 
+    Closed,
+
     Decode(DecodeError),
 
     Encode(EncodeError),
@@ -82,6 +84,8 @@ impl Display for Error {
                 f.write_str("no row returned by a query required to return at least one row")
             }
 
+            Self::Closed => f.write_str("connection or pool was closed"),
+
             Self::Decode(error) => {
                 write!(f, "{}", error)
             }